Look at the cap levels difference between servers.
Ten and eleven would probably be the first to merge.
Eight and ten are so far apart, that there will probably be a new game ready before they would be mergeable, so I would be surprised if enough of us are all still here playing for per blue to justify doing it before they close the game.
There has been a “hard border” in between servers 1-7 and 8+, due to the difference in stamina economy caused by the original orange rarity grind.
This has been apparent in the difference between the amount of deals (and their content!) on S1 and the other servers.
If S8 was to merge with S1, per blue would be giving away so much more resources per dollar to those players than they do now.
While people are still spending on s8, a merge with S1 is not going to ever happen.