What are you doing with your old emails? Do you delete them? Or have you found a method to archive them that produces few folders with few emails in each and that works consistently over the decades?
Because I haven't, and having everything in one folder works just fine with the clients I use.
I sometimes wish to find emails that I couldn't have known I would need again.
Right now, I've been looking for the email address of the landlord of the house I lived in until 2009. I'm happy that I didn't remove his email saying that his son would come on Friday to fix the door handle.
Oh, it's a nonsense, definitely, but it only gets cleaned out every 6 months or thereabouts and I've not yet translated the rules in my cleaning script to Sieve rules to do it automatically.