There is no reasonable argument someone can give for changing their name without having been fully aware the character name they chose was one just meant for trolling or one they had to know they wouldnt like further down the line...
That's a load of utter horse ****. For one example alone, if you are going by naming rules and are playing a Miqo'te sunseeker, there are lore-related reasons to change your last name from Tia to Nunh or vice versa.
Fantasia is obviously another example you yourself listed.
I could name a hundred different completely valid reasons, but I have no doubt that none of them would matter to you because none of them effect you.. yet.
I shouldn't have explain this, though. This is an optional service that will not effect you in any ways, so why the hell are you against it? This
is something I'd like to see the reason for. What's the issue with having name changes available?
If your fear is the irrational idea that people are going to 'abuse' this to take advantage of you, relax and get over it. People already can take advantage of you -- the fact they aren't shows that they aren't going to. Do you really think people want to pay real life money to try and get away with a scam that won't work?
No matter what tiny, insignificant reason you might be against this -- if there is any at all, and this isn't just petty spite for the sake of being spiteful -- it will not compare to the countless multitudes of reasons why people would like this.
Thankfully, I think Square Enix is smart enough to realize this, and that there's a very good chance we'll see this in the future. There's no way to tell how soon, though. Hell, I'm sure that there's money to be made, too, and that they realize this as well.