I think it's important to have some mounts no longer be obtainable Like the Scarab Lord mount or the Glad Mounts. These are more status symbols then anything else. Now the TCG mounts are tricky, While I think some of these mounts should not be obtainable again like the Swift Spectral or the Magic Rooster. Most of the others should be fair game to be Twitch rewards or Trading Post rewards.
With all that said. I do also think they should make available alternate versions of these unobtainable mounts again. Either as a pallet swap or a revamped version. Like With the Scarab Lord mount while i was never able to get that one I was however able to get the blue version from archaeology. What could be cool is adding to timewalking the ability to do the Scarab Lord quest and obtain a pallet swap version of the updated version of the mount, The Qiraji War Tank. Just change the color to blue or red.
This would allow these old unobtainable mounts kinda obtainable again while keeping the original as status symbols.