I fully support allowing the device itself to have alternative OS installations after the end of official support.

The reason 1st gen iPads don’t have this is fairly obvious though - the unlocking feature is itself a likely vector for attacks e.g. by law enforcement unlocking tools, etc.

I’m not sure what it would take to enable it to be done securely without destroying the overall security model but it’s clearly not trivial, and would take some investment that certainly wasn’t possible at the time the 1st gen device was released.