I am not trying to downplay the differences. I legitimately believe that it is a pretty good range. I tend to actually think it is a good thing that as characters raise in levels they get meaningfully more powerful. I also do not look at everything through the prism of Fifth Edition. Not being like Fifth Edition is not a black mark against a game.Saying the monster stays relevant for 13 levels sounds manageable, sounds normal, as if there's no real difference to PF2. But I wager your definition of usefulness is likely a theoretical construct only.
The real take away here is that level bands or ranges are much more narrow in PF2.
Certainly a 5E DM is not used to being told to think twice before having a PL+4 CR monster.
To me the useful thing to say would be use caution, to openly agree PF2 is much more sensitive to level, and that this can trap the unwary GM.
That doesn't make it a worse game. But trying to downplay the difference might.
As far as it being a trap : this is a different game that works differently. The encounter guidelines are pretty clear about how much of a threat higher level monsters are.
I mean the game tells you how it works and does so in detail. When I started running Fifth Edition I had to change the way I designed both combat and noncombat encounters dramatically. I do not consider that a black mark against Fifth Edition. It is just a different game.