PMF is nothing but a framework, a mental model, a bunch of heuristics rather than a magic bullet.

Yes, the ecosystem is constantly changing - but at any given moment in time, you have a specific product targeting a specific market. PMF is a framework to see if you're delivering on your promise to that market in that specific point in time. The corollary to PMF is that the market you've picked is big enough and/or growing.

PMF is not going to save you from external forces (the arrival of the internet in yesteryears or in more contemporary times, the SaaS-ification of everything) happening and neither does it claim to be. That would be an entire different topic ("Innovators Dilemma" - but even then you need to have PMF in the first place to even have that dilemma ;)