I don't think making all the raids valid is the correct move as that just places undue pressure on making tier sets and trinkets that are not too disruptive, which tends to mean fewer fun abilities.
What I would like though is making more old raids into Timewalking, and have them on a rotation so you can gear through several avenues on a more lenient lockout, maybe 2-3 months per raid.
So you do the first raid of 10.0, but you can also get gear from NM and possibly HC versions of Isle of Thunder, which later in the tier swaps to Blackrock Foundry. Next tier maybe it's raids in Firelands and Battle for Dazar'alor?
Also how is Q4 2022 dead in the water? We don't know anything about the expansion at all other than that there will be a later tier that is specifically stated to be designed with less prestige in mind.
A 2022 release isnt off the table until it is either specifically stated, or Alpha/Beta starts much later than what would allow a full development cycle in time for a release.
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I mean, does it? WoD also had an extra "season" at the end, and all that had were recolors of the same appearances from the previous one. No reason to assume this won't be the same and just give another couple unused mount tints.