When is the right time to invest money on Ads?

I know that "conventional internet wisdom" says you should set up a landing page, then send paid traffic to validate the idea but the all process seems dumb to me for a variety of reasons.

First, until you actual collect payments, you aren't validating anything.

Second, people can be interested but doesn't mean you will be able to acquire them at a profit. And if you have no idea about your customers' lifetime value, how do you know if you are paying too much to acquire a new one?

Third, CAC increases overtime, usually a lot, so the fact that a customer today costs you X doesn't mean much. And maybe organic channels don't work for your product, so you aren't able to attract customers in the long-run. (A good way to evaluate if organic can work is to look at google searches volumes for your keyword).

In the end, I believe that the old manual promotion in relevant communities and social media groups is the way to go.