I'm gonna try to cover everything you stated, forgive me if I miss something.
1&2&3) All more or less say the same thing. This falls solely on SE's responsibility on handling this type of inflation.
Inflation comes from one SOLE source in this game, RMT/Bots (which happen to often be one and the same). If SE so chose to be more adamant about the banning of accounts, known buyers and such this can and will work as RMT will not be capable of selling any gil into the economy, hence no inflation via this source.
4) I'm sure you understand what Pay 2 Win means, and as such, buying a subscription for a month or buying a level 50 character is not giving you any significant advantage over others. You do not know how to play your class better then most who grinded through it from the beginning. People who farmed fates back in 2.0 to hit lvl 50 have proven this when many who tried to get into trials, flunked hard and still do. (Ex. Tanks in Garuda EX.) You are missing much of the lore in the game included main story.
For those who are still going the slow grind route? They have chose to do so. They could have jumped a character equally and they chose not to. Say your counter argument was why should they have to? You are correct, but on the same token, whose to say players will still be doing this content forever? I don't wanna hear a flashback run for relic is sustaining these pre 50 dungeons as most of these dungeons have terrible que times, especially for DPS.
If your grind is slow making it up to current content, that can and more then likely in today's generation equally deter people from getting into your game. Many people drop out long before reaching cap.
5) Just because you can buy your way to success does not automatically mean, you can properly utilize the tools given to you. I've seen and still see countless crafters flub on 3 star and 4 star who "bought" there way to 50 from speed leveling services simply because they understand some of the basics of crafting.
6) An MMO example of this is Warframe. From the beginning the game landed, you can buy silver parts (currency in the game) to make every single armor in the game some more broken then others as well as all the same guns found in the game. F2P game as well.
All these same items? Available as drops from dungeons in the game. Two routes to get these items, most players ACTUALLY chose the free route because it was easily obtainable in game. Almost made it feel like you were wasting money by trying to save time. But were breaking into tangents here, were talking about buying a subscription to the game with in game currency and a character jump.
I've yet to also see an example where a character jump item in a cash shop has negatively affected a games lifespan. If anything it's brushed off as a cash shop option, not a vying alternative.