It has failed in some instances, which means it can fail again.

What instances are those? When could you have invested in the US stock market and lost money over 30 years? Or even not made a good return over 30 years?

The US is quite a bit more fragile than it was back then, in no small part thanks to finacialization of the economy and growing debt stocks.

How many times in the last 50 years do you think armchair economists have said "things are different now and what has worked before isn't going to work now?" Sure, there is a chance things are actually different now, but there's no real data to back you up when there is 100 years of data saying the opposite.