It boils down to favoritism and fairness.
As far as gameplay is concerned, the only area with actual exclusives is Feast. Yes, there are other items you can only get through PvP, but they're (as far as we know) available indefinitely. You won't run out of time to get them, you're just not willing to get them if you don't like PvP. Same for raid mounts- and those at least get easier over time to obtain.
Of course there are those who have exclusives tied to purchases too- preorders, item codes.
I guarantee you, because I've seen it, that if you were to propose having Feast items unlocked for the general public through other means than top <bracket> of <specific season>, a huge portion of their relatively tiny demographic will lose their minds. It's frankly surprising they made an exception for the hellhound minions... So what about the mounts? The gear? Are minion collectors more important than mount/glamour collectors? At the time the excuse was Verminion... But where are the item code minions then? Now too we have the fashion thing in Gold Saucer. It's still content that hinges on vanity items, both. Perhaps all exclusives could be banned from these features.
So if it's so unfair to them to strip the exclusivity of their items, it's unfair to strip the exclusivity of any other group's. It's also favoritism to leave other players with specific gameplay interests without something special of their own to work towards.
Expanding or abolishing the concept would both be fair.
This is a sentiment I share as well, although the validity of Legacy status in this context would be touchy, and is very much fueled by one's concept of what it represents. For me, I see FFXIV Online as technically a separate game, and the company allowed them to take items from said game into the sequel, along with special designation for their support of the project despite its hardships. That won't fly with everyone though, and in past threads about this topic, I've seen members state that any arguments against the status quo on exclusive items would have to include Legacy to be considered universally defensible.
That said, I'm very much in the camp of allowing old exclusives - regardless of origin (except Legacy, in my personal opinion) - to be obtainable again... I personally prefer through earned means, as opposed to crafted, if at all possible.
However, if items are to remain permanently exclusive, then I propose adding more exclusives to the game. WoW is an example of a game that has some exclusive item somewhere in its history for just about anything you can imagine, whether it had to do with a purchase or with gameplay. FFXIV is very limiting in this respect, and somewhat hypocritical in that only certain "exclusives" remain so, seemingly so as to pander. For the rest, they eventually become easy revenue on Mog. I don't agree with that.