If there really was a 50/50 chance of it being 200 or 50 then switching definitely has a positive expectation, just like if I paid $100 to flop a coin where heads I get $200, and tails I get $50, that would be a good game and I would play.

But it is not possible for the envelope to have either $200 or $50. It only has one or the other.

I guess the other envelope could be randomly selected, for example pulled out of a bin of envelopes where half were filled with $200 and half with $50.