I really hope they finally listen, it has been such a disastrous change for small to medium sized servers. They used to thrive, but I've watched guild after guild disband due to the roster boss and with each one goes several of its players forever. And I think something people forget is that these servers have been the lifeblood for the larger servers, the stronger players would be recruited by guilds on them and leave, then the smaller servers would go on to produce more would-be hardcore raiders. That just doesn't happen often anymore on my server, the guilds are all dried up.
I remember when Blizzard gave this reason, and went on to create blast furnace, and xhul'horac, and after that you have to admit there have been few bosses where this kind of thing exists. The reason being that the vast majority of guilds absolutely cannot guarantee at least one of every class, most guilds have struggled to maintain their rosters as is without being able to be so choosy. And that's not a them problem, in an ideal world you would be able to do that, but you have to design the game for the people who are actually playing it.
The other big reason was of course the balance between 10 and 25, but I've always found the narrative odd because it's perfectly fine for some bosses to be harder and some to be easier on the respective difficulties. As long as there's some parity in that regard, it doesn't matter whether so and so thinks this boss is harder or whatever, because they're effectively separate difficulties. It's very possible to just respect that each of them requires their own set of skills, most notably 10 man playing into personal responsibility and 25 into organisation and communication.
That 20 man guilds would start dying (more), no doubt that would happen. There would be unfortunate casualties just like when they made the initial switch, but at the end of the day a lot of people prefer 10 man and would instead be doing their preferred mode. I'm sorry to say but if you look at the state of guilds now, it's eerily similar to how 25 man was looking towards the end of MoP but for reasons that aren't because of a smaller group size mode existing. It's really time to change it for the sake of the game.