Precisely.
DCF and DDM at least try to predict the future (Like you say, even conservatively). The P/E does not. The P/E is just a massive opinion poll (“what the buyer is willing to pay”). As Warren Buffet liked to put it:
“A public opinion poll is no substitute for thought.”
And just because we cannot consider all the moving parts that goes into the Value of a company, is no excuse to throw one’s hands up in the air and give up, succumbing to cheap fixes like the P/E. A DCF or a DDM is as close as a human can possibly get to finding out the true Value of a company. Of course, someone like Warren Buffet might to a better DCF than most of us, but the argument still stands.
The question of a company not existing in the future or having bad financials going forward is a question of research and knowledge about an industry. If an investor doesn’t have these, there’s no point in even beginning to Value a company. So that point is moot. I understand that anything can happen in the future. But a good knowledge about the industry, a broad lattice framework and a minimal understanding of Accounting should increase the probabilities of predicting that future. This is why the concept of ‘Circle of Competence’ becomes so important.
Besides, if an investor truly believes that there is no way to understand the real Value of a company, then he should not be investing at all. If current Prices truly reflects the Value of a company, there’s no profit to the made. This is the line of argument in the Efficient Market Hypothesis, which has been debunked by good investors (Even mediocre investors) time and time again.