You are overthinking it. It's just another consumer electronics box. They are all about equal in terms of reliability and most likely performance as well. Routers can have differences (routing speed, wifi performance) but a modem is a modem. Yes, pairing chipset to cabinet is the ideal but everything is built to the same internationally agreed standard. If there is any difference resulting from a mismatch it'll likely be small.
It's barely worth any more thought than you'd put into buying a microwave oven.
Took me a while to choose my microwave oven too
I get your drift in terms of consumer electronics and limited life spans but why? My two current modems are 12 years old and I'll be fine giving them away on freecycle - and they will get taken, no question.
In terms of what limits a modem's life span, what are you talking about? If I get a modem that will also do fibre (not sure I can though) then I envisage it lasting a decade, with luck. Or is there a test for 'aging' a modem/router with a test score that tells me if I should bin it or not?
My current modems - one is a D-Link, the other a Netgear - I have no idea whether they just need better configuring or are old and knackered.