Blizz promised permanent servers for Era well before launch even if there was only “tens of players” on them. Prior to that Blizz had no real interest in relaunching it (let alone TBC/Wrath) and the overwhelming consensus after the ‘Wall of No (classic wow)’ collapsed was that “vanilla” wasn’t going to be popular at all and would be a ghost town inside of three months. Blizz must have had a similar mindset seeing as they were totally unprepared for the launch from a server standpoint.
I bring this up because the OP uses TBC and its impact on Era as a reason they didnt provide TBC Era servers and is likely a reason why we don’t have Wrath Era servers. While Im largely in agreement with this sentiment I would like to point out its possible we don’t have TBC and Wrath Era servers because they never planned on implementing them in the first place.
We got TBC and Wrath because of how popular the relaunch of 1.12 appeared to be and people said they would be totally down for them, not because Blizz planned to release them way in advance.
Wrath might not have been a thing if TBC Classic bombed and most stayed Era. Blizz is just riding the wave right now not operating out of a ten year plan for Classic. They;re just making it up as they go along based on what the current climate is.
But yeah, talking about Era servers for TBC and Wrath it probably would have helped if nearly everyone cloned and played both games instead of kicking Era in the family jewels on the way out the door.