v2 and v3 coëxisted for over three years, giving 16 months advance warning of the deprecation, ending in a four month period where support was removed from the server but the client could still connect to it.
Operators had plenty of time to upgrade their services.
It's not like you can just google an old Onion bookmark that's gone stale to get the current address.
Address longevity is especially important if you're trying to re-democratize the internet and give everyone equal opportunity to host content, not just a handful of mega corporations. In that ecosystem, it wouldn't be out of the ordinary to have 5+ year old bookmarks laying around that you haven't visited in a while but want to check out again. It's a pre-Google internet.