The percentage of hires for companies in that situation is going to be extremely small, yet the "hire for tech stack" motif is cargo culted throughout the industry.

Why else would most companies have open reqs if they don’t see a need for man power soon unless you are one of the four or five big tech companies that are trying to accumulate smart people (tm)?

Because they project for incoming needs. Hiring for today’s needs is a fools errand, it can take a good amount of time to identify a quality candidate. If I need something filled right ie or the company goes under it is already too late.

Doesn’t brooks say that adding manpower to an already late project makes the project later?

who said the project was late? You get a new customer or want to get to market with a new feature fast you still need new people.

On the other hand, you have a few “full stack developers” on hand and you need someone to concentrate on one part of the stack so your current developers can concentrate on other parts.

I’ve found it remarkably easy to hire a front end developer who knows the stack we want to use, who doesn’t know anything about the business besides - here is what we want from the website, give us some mockups and we come up with the APIs you need.