A fair amount of people were fine to let Wenger to because he'd stagnated right below the top 4. Emery lost a lot of support once he botched getting 4th in his first season. And now under inexperienced Arteta, the narrative is "no, you need years to get to 4th." Surely you can see the double standard.
That's because there's a realisation, across the board, across the fan base, that this wasn't going to be a get a new manager, sign a couple of players quick fix.
You said it yourself, the squad had slid under Wenger. Most thought a change of coach & a couple of new players would change fortunes & get us back into top 4.And it looks like it could initially, but then same old problems came & results detioriated. We had Ozil situation, Auba contract situation looming, structure behind the manager still chopping & changing.
Even initially under Arteta, he got a decent start, won the FA Cup, & I think the thought was a couple of signings, guys like Partey & Willian could have them challenging for top 4.
Then we had that awful start to last season, & I think the realisation hit everyone, from boardroom to fan base, that this seems a more deep rooted problem. That this was beyond just a management issue & that a proper squad reset was needed. So, rather than get rid of yet another manager, they've trusted Arteta & Edu to do the squad reset, to go down the road of signing younger players to try & build a new young team to hit their prime together, either that at the very least would be still have resale value if it didn't work out.
Now, is Arteta lucky the Owner backed him & agreed this with him this time last season & kept him in the job? Too right he is. Is Arteta lucky the Owner decided to invest their own money into the team for the first time this summer to allow him to carry out that strategy? Again, too right he is. Will Arteta be good enough to take this team to another level passed this summer when the last of the old guard like Aubameyang & Lacazette will likely have left? Probably not. But at least he's carried out what was badly needed, even if it might take another manager to take it to the next stage.