The problem with your question is that there never really was this kind of situation before. The Steed of Rivendell is only the third mount with an account-wide option that ever was available in the Store. I can only tell you what happened with the other two.
The Reveller's Gilded Steed always had a per-character and an account-wide option, plus there are a per-character and an account-wide bundle which contain the steed plus the Reveller's Gilded cosmetics. All of these are usually available during the Anniversary event and once again in autumn, roughly six months apart from each other. So far, the account-wide options for the Reveller's Gilded Steed have always been included in those returns. The situation differs from the Steed of Rivendell in that the Reveller's Gilded Steed does not include any War-Steed cosmetics (the corresponding set is earned in-game during the Anniversary Event instead) and that no other mount has been offered as part of a mount+cosmetics-bundle yet.
The Steed of Bree originally only had an account-wide option when it was first available in December 2013. It was available again in April 2014 during the Anniversary Event. By then, a per-character option had been added (because players requested it, I think). In this case, the account-wide option was available again, but the starting point had been a slightly different one from the Steed of Rivendell, which arrived at the Store with both a per-character and an account-wide option to begin with.
These are the facts we know of. My personal speculation is that the Steed of Bree, Steed of Rivendell and any future mounts with the same setup (mount plus War-Steed cosmetics with an account-wide option) will return to the Store on a longish circulation (because they are released at a slow pace to begin with) and will always have the account-wide option available (because it seems that Turbine has decided that for the premium-plus mounts the account-wide option is a way to get more sales than with the per-character option alone).
German, female, migrated from CodeMasters in 2011, Premium member, slow-leveller, pure PvEer and opposed to Forced Emotes - wherever I fit into the statistic, it's not in its middle.