Soviet, let me address some of your arguments:
-Yes there are some questionable tech companies around like Twitter and Uber, but then again the most valuable companies in the world are hugely profitable tech companies like Apple, Microsoft, Facebook, Alphabet, Amazon, etc. The number of rubbish tech companies is probably fewer than the last tech boom which ended around 2000/2001. Incidentally the end of the last tech boom was just a mid cycle slowdown and was not enough to stop the bull market (did cause a pullback though) or crash the economy (lower interest rates reflated the economy).
-In regards to Shale oil producers this risk is well known by the market and low oil prices have been here for a while. Its not coming out of left field to surprise the market, so everyone is prepared for the coming losses and it won't crash the market. The other thing with shale is that unlike traditional oil the assets are typically quite short term, so if you cut back on capex, so typically you can simply run down 70-80% of your existing assets (sometimes more) within 3 years. Simply by not spending on capex (i.e. not renewing their shale sources) this will cut out a lot of supply from the market and its already started.
-As for U.S.A. stocks being overvalued I already addressed that previously. Yes they are overvalued and will likely stay overvalued for years to come partly due to low interest rates. Look at history and you will see stocks can remain overvalued for very long periods of time.
-As for subprime loans in the auto industry I think there may be a temporary dip, but longer term as the economy continues to strengthen sales will turnaround. Also subprime cars are hardly a big enough problem to crash the economy. The problem could eventually cause loses to some car manufacturers and niche financiers but car companies or niche lending companies going out of business won't crash the economy. Subprime crashed the economy previously because major banks were at risk of collapsing.
-Subprime student loans if certain niche financiers collapse it will not cause the economy to implode. Also a large part of the losses can be moved onto the government balance sheet. 1.4 trillion of losses is not enough to endanger the government or cause an end to the boom.
-Home prices are naturally outpacing incomes currently because they are recovering from what was a previously depressed cyclical low. If you look at home prices from a longer term perspective they are hardly at nose bleed levels (which they mostly likely will be by the end of the boom).
-Business inventory numbers tend to be very volatile over short periods of time. As for bricks and mortar retail yes that industry is gradually dying. However a lot of the sales lost from bricks and mortar retailers are going to companies like Amazon, so the net impact on the economy will be lower than many people think.
-The wealth gap in the U.S.A (and indeed most developed economies) has widened for decades. This is business as usual. Also household income growth is finally picking up. This article is a little dated but I think the trend is generally underway.
http://www.cnbc.com/2016/09/13/here-are-...ot-up.html
Also if you look at an index like the S&P500 a lot of these companies have other ways to increase profits such as through automation/efficiency and through increasing overseas sales to Asian and Latin American markets. I think capital expenditures will pick up in coming years, as record profits must go somewhere. With rising interest rates (higher borrowing costs) and higher stock prices/valuations reinvestment into capital expenditure is gradually becoming relatively more attractive compared to share buybacks or takeovers than previously.
Alsothe nature of our large corporations has changed we have more information/tech companies than before. If for example G.M. wants to expand they might open another manufacturing plant whereas a tech company like Facebook might make a takeover for Whatsapp and then spend hundreds of millions of dollars on research and development and marketing to improve the business. However the accounting and tax treatment of this kind of investment is such that its not always classified as capital expenditure even though its a real investment into the economy. So structurally a lot of investment no longer shows up in traditional capital expenditure figures, but that does not make it less real.
-As for central banks having missed the window to raise interest rates that is you effectively making an implied call that the next recession is soon, which I think is an unlikely scenario. As for what governments around the world are doing governments and central banks have and always will be incompetent and that is not sufficient reason for the boom to end. Besides in Asia infrastructure investment is booming and I think you will see eventually a boom in U.S.A. infrastructure will happen as the U.S.A. government will look to bolster the state of its crumbling infrastructure. I understand there is a possibility of it getting derailed due to politics but I think infrastructure spending will eventually come through. According to the article there could be a trillion dollars worth of infrastructure coming down the long-term pipeline.
http://www.cnbc.com/2017/04/03/china-wan...-easy.html
-Small business creation is at a 40 year low because the big companies just keep getting bigger.
https://www.economist.com/news/special-r...ing-global
I don't think its enough to stop the boom as the large corporations keep churning out record profits and keep the boom going.
-Look at history debt can keep rising for years. Just look at Japan. Yes, eventually it will get to a breaking point but can you confidently predict the breaking point is soon? I think the breaking point is years away. Can you make a convincing logical argument why the mounting tower of debt will soon topple? There are commentators that have been predicting imminent collapse due to the debt burden literally for decades. Without proper timing this is useless. In most developed markets inflation is already picking up off a low base. The E.U. and Japan are a little behind on the trend but I expect it to turn around into higher inflation sooner rather than later.
Finally just because the boom has been going a long time is not a reason in and of itself for the boom to end. Especially so when you consider that annual average GDP growth during this boom/recovery has been lower than the annual average growth rate of most past recoveries. In other words the recovery/boom has been longer but less intense than most.
Conclusion:
The market always climbs a wall of worry. Be careful of listening to the doom and gloomers. Look at how many years Peter Schiff, Jim Rogers, Mark Faber, etc have been calling for a market crash that has not happened yet. A broken clock is right twice a day. Market crashes occur very infrequently historically speaking and history shows its generally a mistake to be overly cautious and sit on the sidelines waiting for an impending crash. Often people that do this, by the time the crash happens they will have missed out on so much gains that even after the crash they will be worse off then people that just kept investing.