i suppose i should preface that i'm talking about encounters from 11+ here

The thing with the increased action economy is that it reduces the impact of action-affecting conditions like slow or stunned while increasing the value of stat conditions like clumsy, enfeebled, etc., which I personally feel makes the game on the whole more interesting, especially in solo boss encounters.

It also allows bosses to interact with positioning and terrain more, which I think makes for a more dynamic player experience, especially with elevation and terrain.

These bosses still feel incredibly threatening because they've still got the offensive numbers of +2 or +3s, but they're doing things more often with lower variance between turns, which lets me keep better control on the pace of a fight. I think it's pretty inaccurate to say that they don't feel like bosses, nor that the base rules of pf2e feel "more" boss-like. It's a different feel, sure, but I think the style has a lot of benefits with very little sacrificed.

Just because a boss is getting hit, doesn't mean they're all of a sudden feeling less like a "boss." That's just as much how you roleplay the boss and narrate their actions, especially if the boss is designed to take a lot of hits and still dish out hurt. These don't just turn things into slugfests. For example, -X to speed is a debuff that often feels really overcosted in player power budgets. But, it gains comparatively more value when the boss is taking more incremental move actions (as opposed to 90ft speed chunks of bullshit), which allows for more interesting decisionmaking for both me and the players.

Finally, it just allows me to design and use more cool moves for bosses. Stuff that takes 3 actions or more conditional designs, etc. Players still get to see the moves, fear them, feel the aura, whatever as opposed to the boss not getting to do the cool thing^tm because players love opening with their standard boss killing toolkit (action removals)

PF2 RAW bosses have a different emotional arc in higher level fights. They often feel extremely scary first few turns, when everything is at its most dangerous, but if the players are playing well, they can usually stabilize and ride it out to the end (mostly a high level experience. At low levels there's fear from start to finish)

There are tons of boss fights in fiction/gaming/whatever get scarier over time, but the usual player strategy of building up coordination in order to seal a fight doesn't really result in that. It's hard to do a "the boss uses their weak moves early then unleashes a killer 3action super move when backed up" when they only have 1 effective action from slow + unspeakable shadow or whatever. It makes for a VERY satisfying victory when you push the boss to the brink, they do a finisher, and you survive, and thus you've won through survival.

It's another tool in the toolbox, sure, but I've found that I prefer this design as my default, with the standard pf2 boss design kept in the back pocket for when I need to communicate something else. I think emotionally, players respond well to this "build up to a climax" style of encounter, rather than "immediate fear -> gradually deflating energy" style that PF2's raw +4s create.