Regarding Bayern Munich's turnover of managers, I think the issue was with Salihamidzic.
His reign as Sporting Director was from 31 July 2017 to 27 May 2023. During this period of time, excluding Ancelotti who was already in charge and was subsequently sacked by him, there were 1 caretaker + 5 head coaches appointed (inc. Tuchel).
During his predecessor Sammer's reign from 2012 to 2016, Heynckes retired while Guardiola left at the end of his contract.
With Christoph Freund and Max Eberl in charge now, there'll be a "reset" (and I think their record at their previous clubs suggests they should be better at their job than Salihamidzic was). They splashed the cash on Min Jae and Kane and still have pretty good core of players at the club.
As for longevity in the job, there are rare exceptions like Simone and Gasperini (besides Klopp and Guardiola, obviously) but the average tenure seems to be ~3 seasons. Guardiola was at Barca and Bayern for 4 and 3 years respectively. Emery's stay during the successful spell at Sevilla was for 3 years. Ever since AC Milan days (8 years), Ancelotti's longest stay is his current 2nd spell at Real Madrid (3rd year). That hasn't stopped them winning silverwares/doing well wherever they go (i.e. ambitious/talented managers don't/shouldn't worry about job security "that" much, but rather focus on the prospects of achieving success).
Regarding Leverkusen fans' reaction if Alonso were to make the move to Bayern Munich, I think they will factor in (if it happens) winning their 1st ever Bundesliga title (and possibly a double or even treble); him being an ex-player there; being used to seeing Bayern being the "bully" and taking away other teams' best properties.
I do not mean to say Alonso should/will choose Bayern ahead of us, just stating why I won't begrudge him if it does happen.