[philipov]

That's like saying differential equations aren't mainstream just because not everyone takes university-level math. Stacks are a mainstream technical concept in a mainstream technical profession. The relevant domain for determining prevalence is "Programmers." Non-technical people don't count.

[ergothus]

I'd actually say the relevant group is "people trying to understand the domain" (given the question in the headline).

Which, generally, boils down to a group that is largely programmers, and even those that aren't programmers DO (AFAICT) have familiarity with the "stack" concept (non-technical Eng Managers, non-technical PMs, etc)

I'm not saying you're wrong in any way, just pointing out that it can be said in a way that doesn't sound like "programmers are all that matter" (which I don't think was your intent anyway).