Will gach, cubes etc. soon become illegal?

So what you're suggesting is I could use my Mesos at a store like a Debit card?
Or how about a TF2 hat? Just because someone would pay real physical money for some virtual item, due to rarity, exclusivity or some other nonsense, DOES NOT give it value in the real world.

You also couldn't hand over Canadian dollars in most US stores and expect to buy anything, does this mean canadian money is only real in Canada?
No, it means just like any currency you have to go through an exchanger to make it compatible with the current locale.
You can sell your mesos to meso buying sites (or other players) for real world cash which you can then use for real world things, and in return the meso selling sites sell your mesos for even more cash to others.

BitCoin is an absolute fiction but it has been used for real world transactions in the same way. Currency is currency; As long as someone values it, it can be traded for other things and someone will find a way to exchange it somewhere. Money changing is the 2nd oldest profession.