I can't see any agreement to a play-off between the Championship winner & 12th Place Premiership team. Exeter are pretty much the only team who were promoted to the Premiership during the years when Championship playoffs were in place that managed to kick on. All the others have tended to yo-yo between the bottom few places in the Prem & the top of the Championship.
This is partly down to the fact that if you don't know if you're coming up until the end of the season, it's hard to put together a team that can compete. Exeter managed it by dint of planning for the long term & having financial backing that was willing to pay what was needed.
Before you say "Ah, but look at Us, or Saints, or Bristol this year", all those teams knew they were promoted well before the end of the season & could start planning, talent scouting etc when players were on the market.