[begueradj]

> "Like it would make sense to have resistance to one or two things, but all those things? "

It makes sens, and all organisms and creatures would be more than happy to evolve to where they have resistance to everything.

That's a way to remain healthy, strong and spread own genes as much as possible for a long time.

[jajko]

There is usually some energy cost associated to keeping resistance unless critically needed. Or limited place on genome.

Any organism from virus to human will happily shed over time (generations) any resistance we now consider crucial to survival, just change its environment enough.