I get there are probably technical limitations making it difficult to swap a tamed elk from livestock to mount, but regardless I still find it very frustrating that tame elk are an entirely different entity from wild elk. I don't know about the rest of you, but making elk an end-game thing feels like it flat out prevents certain playstyles from ever getting to use them in any way. I can't imagine a Nomad managing to gear up to take on the archives and then saving the gears to buy the elk, for example. Additionally, once you can buy an elk, it likely isn't as useful as it could be earlier on. And finally, I feel like gating 2/3 of transportation options behind the final boss is just a really weird choice.
Instead, I would suggest maybe the ability to put a leather saddle on a domesticated elk, which would essentially transform the elk from livestock form to mount form. This way it still requires some effort (gathering leather and domesticating), without gating it off from people that don't want to sit in one place for too long. I don't think this would make it THAT much easier for nomad playstyles, but I feel it makes more sense than all wild elk being completely different entities from the tamed ones. Perhaps one could use pelts instead for cheap, nomadic friendly saddles to ride the more wild ones, and they just don't listen as much as the domesticated ones? I'm not sure, just spitballing here.
If they've posted somewhere why they've decided to go with this thought and I've just missed it I apologize.