It's currently opt-in, because a significant percentage of the user base is not running an OS version that can support the E2EE features.

When that percentage is high enough (a few years), I don't see why Apple wouldn't make it opt-out. (Default it to encrypted, you need to specifically disable it if you don't want it).

Wouldn't the easier option just be to make it actually enabled for all devices that can support it.

Why does what % of the user base who's running the support OS versions make any difference?

Then people will complain about their lost data. This will stay opt-in for a long time.