TBC era recruitment happened because of gearing restrictions and that if you wanted to completely fresh players or a coming back one, you would have to spend tons of time to gear them up - this isn't the case now.
As for server progression. I'm quite convinced, that if you asked Mythic community what they care more about: realm progression/identity or easier recruitment the vast majority will chose the 2nd option. For example Polish raiding community was for a long time more interested with Polish guilds ranking rather than server rankings. Nowadays, all Polish Mythic guilds has been forced on EU-Burning Legion Horde side anyway. We were the last guild that tried to stay Alliance and smaller server.
I would assume for international community it matters slightly less since they can chose between few more servers. But with smaller sub numbers, the situation will get worse before it get better when it comes to recruitment within your own server.
The others solution would be to make transfers free if you haven't transfered for longer periods of time. Having to pay money to transfer to a low pop realm and then realize that the guild didn't work out prevents a lot of people of trying to find the guild most suitable for them. Instead, what happens is that people focus on few realm/faction combinations with decent raiding population and the majority of servers die. And connecting them doesn't work at all. My old realm got connected at the end of MoP and nothing has changed for any better except for AH becoming more messy :P So while my guild really enjoyed the server itself (it was nice and peacefull) and most of us were long time Alliance players, we had to transfer to another server and change faction in order to continue to exist. While there were people potentially interested in joining us, having to change both realm and often faction stopped them from it. Afaik, noone on our old server is doing Mythic raids at this point.It'd be great for dead/dying realms who have trouble fetching the requisite number of skilled players for high-end Mythic raiding but I think there are solutions to the problem which do not involve cross-realm Mythic off the bat.
So basicly, I would welcome any change that would help people play with people they want to play w/o worrying about the transfer/faction change costs (in some countries this fee is quite significant)