I think a bad guy failing a save against a spell and ending a fight is pretty darn fun, just like failing a save against a Rogue using Knock Out is fun, or a Fighter rolling a 20 with a Vorpal Blade once or twice every fight is fun.
Legendary resistance should not logically make high-level games unattractive for martials. While enemies can use Legendary resistance to prevent things like Topple or Knock Out or Stunning Strike, martial classes still have a lot they can do that bypasses saves all together.
