How about instead of update_meta, we use meta set?

wp user meta set 123 key value

It would be more consistent with wp option set key value.

wp user meta set sounds good to me, but is there a straightforward way to add a sub-sub-command? (Where user is the command, meta is the sub-command, and set is the sub-sub-command)

You can treat 'set' as just another parameter to the 'meta' sub-command.

The alternative would be to introduce a user-meta command:

wp user-meta set 123 key value

wp user-meta get 123 key

In this case, we could have a helper class which we could reuse for post-meta etc.

You can treat 'set' as just another parameter to the 'meta' sub-command.

True, but then you don't get the nice error messages for free when you leave off the sub-sub-command, like you do when you leave off a sub-command. You'd have to implement those messages yourself, which doesn't seem like a great solution.

Adding a user-meta command feels slightly neater in terms of implementation, but not quite as nice when you're calling code. Might be the simplest solution though.

Even if we implemented sub-sub-commands, we'd still likely need them to be in a separate class, so it's worth having user-meta as a first step.

I merged everything except the --url commit.