The problem with waiting until it's totally and completely borked is then you have to panic scramble to get another phone instead of planning out a purchase. And I had difficulty doing this because my bank wanted to verify it was me making a large purchase...but my phone wouldn't work so I couldn't authenticate myself because that's how everyone authenticates themselves. It's a whole ordeal.

Granted, this can even theoretically happen with a new phone if you drop it or something. But at least with an old one, there are often signs before it goes completely kaput. My beloved cheap ass Motorola phone would bootloop occasionally and started doing it more and more often toward the end of its life. One morning before work it just totally died and would never turn on again, not even able to get the boot screen. (Troubleshooting revealed unfixable eMMC failure.) That was a fun couple of days trying to figure out how to buy a more expensive item without authenticating myself on my phone lol.

I usually sell my old phone and buy a new to me used phone. The cost for upgrading is usually only $100-150 for me and I typically end up getting a pretty massive upgrade hardware wise. I haven't had a brand new phone in over a decade now.