Lets talk about the economy

Strictly speaking, yes. But the buying power would shift tremendously. It's not like a 90% meso cut also equals a universal 90% price cut in the free market. Those with less mesos would be affected to a much lesser degree, and thereby gain more purchasing power, while those in the higher bracket would essentially become "average joes".

You're right about it not being a universal price change in FM, but that's not a reason why. People aren't affected to a lesser degree, they all lose the same percentage of their mesos. Any interpretation of what makes someone "higher bracket" or "average joe" would change accordingly.

In addition to what people are willing to pay for things affecting prices, there are built in price floors (or ceilings) for things that you can buy from NPCs. If they wiped out 90% of everyone's mesos, those things would cost 10x more in relative terms - things like profession ingredients, mounts, and so forth. These, and the things made from them, are the things that wouldn't change in the same proportion as the rest of fm from a meso wipe.

I would actually agree with increasing the relative cost of these things. I remember when pot costs were a significant factor in figuring out worthwhile training. Now it's a joke. Money shouldn't just be for vanity and broken but unnecessary gear, it should be meaningful to the entire population, and it should be possible to make it in significant enough quantities to make playing the game financially desirable. A large % meso wipe would make the mesos you get from training relatively more valuable, and the things you could make with professions relatively more valuable. Those are good things.

Of course while there are still huge numbers of hackers and botters they'll still make the overwhelming majority of the new mesos, so this sort of thing really only makes sense to clean up the mess after the exploits are patched.