Huh, no. The other explanation is that PF1 was only popular because 3.5 crashed down and the 4e alternative was unpalatable (for many reasons that are way beyond the scope of this thread). And this is easily proven by the rapid death of PF1 as soon as 5e came in, and the fact that PF2 never took of.
Where does this come from ?
Oh so you are talking about THOSE designers, those of PF ?
Anyhow, the main problem is that your definition of well-designed (as in, the rules avoid causing problems during use) is not a commonly accepted one either. For me, quality is whether it satisfies the customer, nothing more, nothing less. And in that respect, 5e has a aaayyyway higher level of quality than all previous editions including PF, as a proof it satisfies very well way more customers. Previous editions is what is referred to as gold-plating, over-design which in the end hampers customers satisfaction by being over-complicated. Android vs. Apple is a very good example of this as well.
And despite what you may be thinking, it's way more difficult to achieve something simple and extremely usable than to achieve something extremely complex. I don't know if you've tried it personally, but I've tried it in engineering for decades and only achieved it a few times.
And yet, it was extremely well made for its time, groundbreaking in some way even if it was not perfect. Nobody says 5e is perfect, but it is of way higher quality than systems like PF2 which very few people even want to read, much less play.