The size of the PvP community is not solely determined by the number of winners... And what are you comparing to? Total players, Characters created, active players at max level, active players at max level who cleared savage ?
Lets compare PvP Ranked community vs. PvE Savage Community
PvP community isn't big, but there are probably 500+ people per dc at the start competing, probably half stopping within the first month. If you compare it with PvE, after 3 months of e12s being released, an EU datacenter has still under 1000 people with the mount. NA is a bit better, and JP way better at clearing.
You can argue now, if PvE Savage content is only for single digit % of the community, they shouldn't be mad if it dies, right? Even more so for ultimate. xD
And as far as I remember, they only took mounts that are part of mainstory or really cheap, and recolored them for mogstation. So it is not like they are exclusive or hard to get to begin with
I see it like, getting the reward is like a trophy or medal from a sport competition, you wouldn't want anybody else to get your 2020 Olympic Gold Medal if you won it by yourself or? Athletes simply enjoy that they won, they don't think about how they prevented others from getting it. But they would care if suddenly everyone gets a 2020 Gold Medal, for doing the same you did but way worse x1000.
Rank Decay doesn't fit this mode, once you understand it more, you will realize it. And it is also not the reason why it dies within 2-3 weeks at lower tiers. Just start simple by asking yourself, how you would implement rank decay; how often do you have to play to prevent decay from happening? Will there be still max rating then? If not, would people not be more pressured into playing the least possible just so they can stay around their peak rating longer, instead of risking many matches and losing it all. Another issue is that the season may be long, if you invest your time at the start - it means you will lose your rating till the end because of decay. But if you start towards the end, you have no disadvantages.
Decay would also hit all the casual PvP people who just want to reach a certain tier for wolf collars.
A better way would be, to promote/reward something like a daily Feast roulette, that has a +/-0.5 rating modifier. So you could gain 50% more rating with a win, or lose 50% less in case of a loss. Or make wolfcollars drop like fenrir pups within the mode randomly.