I think the question roams down to what people prefer than what is the wisest and most beneficial decision. I don't think there's that many 1.23 players that couldn't access a TSF server if needed be, I simply think they choose not to because the expansion set was in itself kind of stupid and there wasn't much of a reason to stop playing on the same server as your friends did for the sake of a tile set increase and an unfunctional character that didn't even support proper customization.

That’s the problem with any online game that receives an expansion in same likeness. If you look at WarCraft 3, 30% of WC3’s battle.net games still occur on the original game. I don’t think it’s ever going to change because you can’t force people to play your preference.