People already buy coil runs, and people already can buy crafted i110 gear, which will be obsolete by the time the next major patch comes(and since Yoshi-p says he's not doing that again, then that's less of an option). So adding a way for poor people to KEEP PLAYING THE GAME would not affect anything.
Other reasons I saw mentioned:
-Can buy endgame crafting stuff? Because a person selling Chronoscrolls is sitting on all 50s in DoH/DoL amirite? if that were the case, then I would assume they'd already be working towards that goal. But besides that, "Wow more crafters crafting stuff for the market!?!? SOOO BADD OMG!!"
-vanity stuff? you mean the same vanity stuff SE has in their cash shop in a P2P game? since selling vanity stuff isn't considered p2w, then obviously buying vanity stuff can't be considered p2w either. Oh and TERA also did the P2P with CS before they went F2P too, so since that was a 'failure' according to some, maybe SE should stop doing that too?
-bribing other players? up to the player to decide if he wants to take the bribe or not, nothing to do with the economy.
-housing? that's a game design issue. if there were more plots available for more people, it would be less competitive! even now you can save up for weeks, only for that house to be sold to someone richer. that's Eorzean life for you.
-RMT has already been discussed into the ground so no need to bring that up again.
In the end it just seems to come down to people not cool with the fact that richer folks in real life, could get fake money by using their actual hard earned wealth, to supply other people with more gameplay. Which would also mean less people having to drop their active subscriptions due to real world concerns. All because some people could buy coil runs(people are already doing this) and crafted gear? What are they gonna use the crafted gear on? The raids they already bought? For vanity(oh noes!)? PVPLOL? Or maybe to do raids with their statics, since the gear doesn't carry you through mechanics.