I feel with how the game is now, it would make sense to replace the Pikachu in the Sandy Shallows secret room with a more updated Alolan Raichu. Archford is a more difficult region to reach, and the event Pikachu feels outdated in comparison. It seems to make sense from a balancing and inclusion standpoint.
It was labeled as planned in Make Sandy Shallows Pikachu evolve into Alolan Raichu, but I don’t know if that is still the case. Probably still awaiting evolution editor changes?