> The down-side is that if you decide to move registrars, that still complicates things. What if the new registrar refuses to issue a new cert without a hefty fee?

Then everyone stops using that registrar and they go out of business.

> Or what about revoking the previous cert?

You're asking this as if there is some kind of functioning method of revoking certificates already. If anything this makes it easier because it could be plausible for clients to somehow retrieve who the registrar is for the domain and then only accept certificates signed by that registrar.