It's not like support for v2 was removed immediately after v3 was released.
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.
Yet, the clear-web DNS entries of major websites haven't changed in 15 years or more.
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.
In reality though, most onion sites don't even stick around for very long anyway. And the URL scheme has nothing to do with it. Most sites are either illegal in nature and so regularly disappear for various reasons(police, exit scams etc), or they're just onion versions of clearnet sites, which means they'll be easy to find the new URL for. Or they just stop working because they cease to be maintained.
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.