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 ;)
The fundamental premise of the framework is that PMF is a discrete event and the journey of a company can be thought of as pre PMF and post PMF. The reality is that most founders will be unable to articulate when and if PMF has been achieved.
A journey of a startup to scale is better categorized as a continuous series of PMF points where the product evolves to target a continuously changing perception of the target market in the eyes of the founders.
Obsessing over PMF as a fundamental metric will limit founders from exploring the true complexity of the ecosystem in which a startup operates in and can often be a stumbling block in optimising for their subjective outcomes.
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 ;)