When you say 'overtourism' you mean absolute numbers. If you want to restrict tourists you can just limit them. You are trying to do that with a tax, but just by setting a quota the same goal is accomplished.
Anyway, Bhutan has decided it doesn't want poor tourists and has a minimum price one has to pay daily, in order to get a visa. It, thus, is not overrun with tourists.
And this has nothing to do with an airport tax.
If VCE has a high airport tax, why wouldn't someone just fly in to another city and take the bus or rail over?