To prevent "I don't like it because it's unexpected and/or new" feedback we could have our changes to the Special:Block UI for Partial Blocks be opt-in for a period of time before becoming the default. This could be a Labs feature of a separate preference. We'd also need a way for users to submit feedback and opt back out of the new UI.
Note that this would only affect an individual user's display of Special:Block, not the availability of Partial Blocks or the UI of anyone else's site.