If Villa win: They are already in the top 5. Depending on the final math, their "winner" spot might just merge with their "league" spot, which doesn't necessarily add a new team to the European roster—it just settles which "bucket" they fall into.
If Forest win: Because they are 16th, they are a "pure addition." They occupy a space that wouldn't otherwise exist for an English club.
Summary: A Forest win is the "chaos" scenario that maximizes English representation. It effectively rewards their cup run without punishing the teams that performed better over the 38-game season.