If they turned it into a museum, they'd still have to maintain it as well as the new building in Slough that they'd move into. Also our prestige would lower, state opening of parliament might not be able to happen and thus another portion of tourist income down the drain also. I think there's more of an appeal to visiting it anyway as it's a functional building.

The money for a new permanent building would probably come close to £420 million anyway. The Scottish Parliament alone cost £414 million back in 2004

And that building doesn't have much of a tourism draw... Except maybe the nice pools

Slough.