A year after Morrowind's release, it became part of ESO+ and was made available in the Crown Store, but without the Warden class, the pre-order & collector's goodies. Those are available to be purchased separately, Warden class upgrade and a pack containing both the pre-order & collector's edition goodies. It seems that Summerset is receiving the same treatment, and probably Elsweyr will receive it too.
A year after Morrowind's release, it became part of ESO+ and was made available in the Crown Store, but without the Warden class, the pre-order & collector's goodies. Those are available to be purchased separately, Warden class upgrade and a pack containing both the pre-order & collector's edition goodies. It seems that Summerset is receiving the same treatment, and probably Elsweyr will receive it too.
It is totally irrelevant that the zones will become a DLC after a year since they already bought it. You cannot by a DLC that you already own. Not to mention your information seems incorrect to begin with as there does not seem to be any mounts, pets, et al, bundled with the DLCs of the former chapters, but again, immaterial to this thread as OP already owns it so will be unable to buy it again.
To OP, I suggest opening a ticket with CS. There may be something they can do. @ZOS_GinaBruno @ZOS_BillE might be able to help or something as they are official messengers for Zos.