I don’t mind that it’s a choice, but I think legendary resistance is just boring. It serves an important role because taking out an ostensibly legendary monster with a single save-or-suck effect is very anticlimactic. But, there’s no flavor to legendary resistance. The DM just gets to say “nope” to a failed save 3 times. Heck, if the DM doesn’t directly tell the players the monster failed but it’s using a legendary resistance, it might even be completely invisible. But if they do that, it makes the player feel like the action was wasted. I much prefer something visible, flavorful, and ideally, specific to the monster.
For example, in my custom Strahd stat block, I replaced legendary resistance with the ability to turn into a swarm of bats as a reaction to getting hit with an attack or failing a save. A swarm of bats is immune to most negative conditions, and generally difficult to pin down. So, he can still mitigate the impact of a save-or-suck spell. But, doing so eats up his reaction for the round, and his action on his turn, since turning back into his human form takes a full action to do. You don’t get to restrain or charm him, but you do get to waste his action economy, so there’s still a lot of value in targeting him with such effects.