Well then what is a good reason to change if not set of features offered at one point in time? I guess one never changes unless the brand dies, is that the idea?
If that's the case then I'll beg to differ. I changed twice, in 36 years of photography (maybe that puts me in the rushed and irrational category, I don't know
). My first time was about 1992 I think (could be +/- a year or so) when I left Minolta's MF system to switch to Canon's just introduced EOS 5 (the original) with it's incredible (to me) AF system. I am glad I did, Minolta didn't catch up to anybody until a few years ago when Sony released the A9... Minolta had AF bodies in 1992 - they just were headed in a weird direction that I just didn't want to get into.
That switch gave me 20+ years of awesome photography with Canon - Eos 5, Eos 50e, 1N, Eos 3, 1V, 20D, 1DS, 1D mk4 - that system gave me an absolute blast but in 2015 it was time to switch to Nikon and best decision ever (and costly too) but at the time, image quality from the Canon bodies was just average at best. I went from a 5D mk3 and 1Dmk4 to a D750 and I still thought I was in heaven. Even the AF acquisition was better, and image quality was in a different league altogether. In my view, Canon did not release anything competitive until the R5. That would have been 7 years of less enjoyable photography if I hadn't switched (and yes the 5Dmk4 is an absolutely useable camera that I could have used to get great pictures but it still doesn't hold a candle to a D850).
So now is the same decision point again - can Nikon close the gap back to make an equally performing
wildlife action system (the only gap they have - if I didn't do wildlife I'd be happy with a z7ii and would not look back) within 12 months? Within 24 months? Within 7 years (like Canon)? Within 20 years (like Minolta) and am i happy enough shooting Nikon while they close that gap. Can they get there with something more affordable than the Z9? How soon?
I see this idea that camera makers leap-frog each other all the time and I actually think it's not the case when you look at the full system. Yes the 1Dx and D4s/5/6 played leapfrogging for the past 7+ years and the Z9 when it is released will very likely leapfrog the R3 (not so sure about the A1)... but not the core camera line. Canon hasn't put out any body that competed with the heart of Nikon's line in a decade. They finally did with the R5, and what a competitor it is.
In the meantime, while the other 2 were navel gazing Sony built the most complete mirrorless system and their feature set at various price points is well thought-out - nobody comes close except Nikon with their dSLR line and Z6/7ii lines but it takes both to match it, with all associated challenges of having to shoot dual systems and the fact that dSLRs come short on some key features.
Long answer to say, there are times when systems just have inherent advantages. Those last longer than the few months leapfrogging because they are a reflection of long term R&D investment decisions (Minolta's "strategy"; Canon's in house sensors vs Nikon going all in with Sony etc...). I agree that "who has the best bird eye AF today" is a very fleeting, short-lived lead. Who has the best platform (including glass, flash, image quality, AI capabilities etc...) to lead the long term transition to mirrorless is neither short-term nor fleeting; who that company will be though is not as clear-cut as the current situation might lead to believe.