Just a thought on situations like this that tends to upset the majority of people who strived to get the mount for the Meta achievements.
Just give those folks that did these Metas during content a title that you cannot get once the current expansion is no longer current. It was done in Shadowlands with the Veilstrider title. They released the mount for those who completed the Meta after but not the title. It still still gives folks a sense of achievement and a title to show off that they completed it during content when things are a little tougher to accomplish. I think this would solve most of the resentment folks have for situations like this while still giving folks a chance to get the mount that comes with the Meta.