I must correct myself. In my opinion, the real reason Splash was re themed was because Slash became too expensive to maintain, replacement parts either no longer exist and needed to be custom made which is very expensive.
In my opinion, Disney killed two birds with one stone. They knew Splash was getting too expensive to maintain and in the name of social justice they needed to erase Splash, so here we are.
But I think maintenance costs were the real reason camouflaged by social justice.
It makes sense because that is how business works. Their E-ticket needs the revamp and water rides are notoriously rough to upkeep, in particular because Disney neglects their infastructure.
Toy Story is a property used aplenty, nothing else really force like Cars so this choice fits except a Princess Tiana exposure.
A lot of us probably remember when Big Thunder was in such rough shape and there were proposed plans to utilize The Lone Ranger if it was a hit for synergy and cost benefit of the retracking that needed to be done with a refurb.