If they merge / connect servers, the following issues need to be solved first.
- Names
Who decides who will keep their names? What about clones, that some day may or may not join the realms. What if TBC players become bored and now all want back to their original realms? We could end up with tons of duplicate names and too many players, like on release of Classic.
- Spawntimes
No matter if flowers or rares, but also world bosses. On a single realm this is manageable, as very seldom everyone is online at the same time and you can have turns too as you know each other.
Lets assume, the players of 10, maybe 15 realms will now be merged, then we would have a much higher compeition for goods than in the past. Especially if one of these realms does have a bot army or somekind of powergamer guild. Everyone else would no longer be able to find anything, as the spawn rates are not made for this type of competition.
Having to fight for a black lotus spawn can already be tough, now imagine this on connected / merged realms. This would be a nightmare.
But also the fishing event where right now maybe 50 players compete, could become a 500 players battlefield, again giving those in large groups a major advantage.
Some of us players might play on a casual realm, where we can casually collect things. I for sure wouldnt enjoy being thrown into such a competition and it would maybe lead to more players leaving than a skipped raid or dungeon group.
- World Bosses
At Vanilla, world bosses dont spawn right after they died. Instead, they are on a very long cooldown of several days. This limits massivly how many guilds can kill a world boss each week. On my realm, guilds took turns, which worked fine as only 2-3 were actually geared enough.
On connected / merged realms we will not only have a few guilds but many, I would imagine that at least one guild on each relam will remain active, either by clones or not transfering to TBC. This would lead to a massive inflation of potential guilds that attempt the world bosses each week and there is no way that this would work out, unless the respawn system is massivly altered which would be against the spirit of classic.
- pvp
Besides the obvious issue with world pvp that we see on retail, where people hop realms all the time, the Vanilla pvp system allowed 4 players towards the end to go for Rank 14. This works fine on a realm with a limited amount of players. By connecting or merging realms, the amount of players competiting would also increase, so that the cap of 4 might no longer work.
Needless to say, while in the past one dude or 4 dudes blocking rank 14 on a single realm, this could now happen on all realms due the merge.
We need to have multiple realms to avoid rude players, again, on my realm we never had something like that, I would prefer it to remain like that.
- Questing
Due the nature of connected realms, Blizzard added a group tag system, but also upped the spawn times. We need to ask ourselves, if we truly want to go that path now as well, as it will be coming if we merge our playerbase to a few realms.
TLDR:
I did choose to play Classic as we had no connected / merged realms. For me this is a key point of my motivation to play. I enjoy selling my goods, I enjoy knowing my buyers and I enjoy knowing that at 5 pm my turn on picking flowers is. Its a community and I prefer not to lose this, just because we lack a system to build groups for raids or dungeons with people outside of our realms.
Here it would be much better I think to add a tool that allows you to meet players from other realms inside a raid or dungeon, so those that are afraid to lack members for their raid, can find others in such a tool.
The realms, however, should remain their own economy, you dont need a few hundred people to do a quest, level to 60 or do dungeons. Only the 40 player raids could become an issue, but this can be solved by a less destructive way than merges or connected realms.
Besides that, I am confident that those who remain on a realm, will form a new guild and just keep going.