So let's work this out logically. A char with every job at 50 jumps over and starts harvesting and crafting level 50 gear/weapons/consumables, that is the worry? So, who is going to buy all this level 50 gear/weapons/consumables? There will be no level 50's around to buy it. There will also be a lack of gil in the economy to pay over-inflated prices. Give it two weeks (if that) and there will be level 50 crafters on non-Legacy servers, there will be people with millions of gil, the only advantage a Legacy player really has is that they can start doing end game content, farming up items that they can really only sell to other legacy players (at least for 2 weeks).
Who said he would produce LV 50 gear? All he has to do is to HQ everything between 1 and 30.