You know, the more I think about it - the more I shake my head at what they did all those decades ago.
Nothing really went right with the Lagoon, when you look at the original intentions. They really did think that by flooding that swamp they would be able to create their own artificial paradise that would seamlessly connect to actual naturally occurring and existing eco-systems.
You are right - originally, they allowed swimming. Until they discovered the deadly brain-eating bacteria. They also intended it for fishing - they stocked it with fish, and at different times allowed fishing (though I don't know why this was scratched - anyone?). Then we can't forget the nearly mythical (but very real) wave machine, intended for surfing and such, which they had to shut down because apparently no one heard of beach erosion (or didn't realize how a fake beach would be especially susceptible to it).
So all of this just so the MK can now have a 14' deep moat around the front of it.
I honestly wouldn't be surprised if long term planning is looking at somehow enclosing at least parts of it and making a real fake waterway, or something else which seems unthinkable (drain and pave?) but really, long term, is probably going to have to be done sooner or later.