You are still severely underestimating the dedication related to PvP by comparing it to PvE. It takes less persistent effort to get the mounts in Ex fights than it does in PvP. Top 100 PvP rewards take an entire season to get assuming you are consistently winning battles that your rank is high enough (and a season lasts for half a patch cycle, so all that time is spent on running ranked PvP over and over again). PvP is more similar to spending all the time dedicating to get the Firmament titles for crafters, except this is spending all that time on combat and for a much longer period of time. It's not similar to PvE where you have unlimited time to tackle the trial whenever you want to attempt for the mount by putting a PF up. Ranked Matches only occur during the Feast Season (limited period). Saying everyone should get the hellhound just because 4 years passed is like saying everyone should get "Saint of the Firmament" Title / "Beatus of the Firmament" Title after a couple of years when the actual Ishgard Restoration period is over, they didn't actually contribute to restoring it and only the top 10 players of the skybuilder's ranking / top 100 players have those titles. They didn't experience the allnighters, the focused time dedication, and burnouts these players put to reach the top rankings. I agree that there should be a reskinned or recolored hellhound that should be more obtainable for everyone for those who would like a hellhound mount, but it shouldn't be the same hellhound that the top 100 has achieved. To do that is to stomp on all the effort they put into the season.
Farming for a mount in PvE is honestly a lot simpler. Figure out the pattern of the fight, be able to respond accordingly, then farm on repeat, which can take from a day to a couple of months with the help of a guide, but even if you don't get it in one successful run, at least you get a totem which will guarantee you the mount in 99 runs. There is a limit to how much effort you have to expend, but you can take as much breaks in between. In reality, we don't need to wait for an entire expansion to unsync content. Even as ilvl increases, a competent PvE player that has good DPS can already skip a lot of repeating mechanics than when it was first released (Ex: Hades Elegy), so we naturally have an easier time the longer it takes to farm. PvP doesn't have that - everyone is always synced to remove gear/stat advantage, and it only gets more difficult because you're fighting more competent players as time passes the higher rank you go. Every player plays differently, so there's no 'farm on repeat' mode either. Some people may never even advance past a certain rank - and that's the reality of anything related to combat rankings. It's a system that indicates whether you can stand toe-to-toe with others, far exceed their tactical ability, or need to improve because your ability is too below theirs. Having rank decay doesn't do anything to change the top 100 rankings very much in the end because people at Platinum and Diamond will not match with people at Bronze since the matchmaking queue only allows people at or 1 rank above/below your current placement.
Again, the current problem with PvP Queue is that there isn't enough of an incentive to continue playing since most of the rewards are either very difficult to obtain (having to be top 100 or top 10 for 10 wolf collars so most people are excluded). Doing PvP for a long time to get only 3 - 7 wolf collars doesn't give much incentive if there's nothing you want to exchange because then you have to wait for ANOTHER season to repeat the same hurdle to get enough wolf collars, and most people don't have that kind of patience. This can be circumvented with giving more rewards by achieving certain milestones by the end of the season in addition to the collars - ex: reaching gold rank gives gold colored mount, reaching platinum gives platinum colored mount. Reaching silver gives silver colored mount, etc. People are always more incentivized to try to climb if there is a tangible reward that is more readily achievable and this pretty much gives another chance at the reskinned colored mounts and having extra collars let them try to get a previous season reward as well.