This is gonna be tricky we do multiple posts individually, we may end up taking up the entire thread, so I'll put both in one post.
I'd argue that pretty much everything you've written about Klopp and Arteta, and the good progress they made before it all came together, applies to ETH too. The style that Klopp tried to introduce is very similar to the style that Ten Hag is trying to, the high press to control the game off the ball, and creating opportunities by winning the ball while they opposition is unbalanced. The plan is there, we just haven't mastered it yet.
As for Arteta clearing out the egos, and some of the older players, and the idea that ETH is molly coddling, that's not at all accurate. He's had to put down a mutiny from Ronaldo, another one from Sancho, has cleared out a bunch of other players who were offering nothing or were known dressing room leaks, and has improved the application of others. He's also bringing through younger players alongside experience.
For me, the reasons why Klopp and Arteta got time early on are for the exact same reasons that Ten Hag deserves time, he's trying to replace a culture of entitlement with one of commitment, a mid block counter attacking possession averse style with a proactive high pressure one, and like them has taken several steps in the right direction.
I think you have a very low bar for "playing good football" if you're including Eddie Howe at Newcastle, who again just to be clear, played the lowest amount of football per match under him until the rules were changed to stop them timewasting. If De Zerbi were to come in and "just drill what he's done at Brighton" we'd be awful, because they were already a well drilled team that were comfortable in possession, having been built towards that for a few years under Potter before he got there. De Zerbi would need to do Potter's 3 years of foundational work, along with his year of building on top of that. And that's assuming the huge egos at the club would pay any attention to him, given how little he's done throughout his career.
The issue with your last sentence is that it isn't being forgotten. I'll back any manager we get in, but if we keep chopping and changing we won't get anywhere. We need to start putting more thought into our managerial appointments beyond taking a punt on giving somebody else a chance. Until we do, we won't get anywhere magnificent, we'll be stuck in the same boom-bust cycle of the previous decade.