Absolutely.
Used to play on Firetree US Alliance. There were two 25m semi-competitive guilds left at the end of Cata, both of which were forced to move to 10m to progress in heroic DS due to recruitment issues. Firetree has been dead on alliance since wrath. Both of these guilds came together and left the server in advance of MOP, and now have a solid 25m on Area 52, with no issues recruiting when needed! Firetree alliance was devastated as a result of the top two guilds leaving, at this point there is 1 25m left on the faction and it is not competitive by any means.
It was a sad day when I transfered off the server and I still miss it. I started there and played from Vanilla to Cata, I still miss it. However, it just wasn't a viable option to raid 25m there anymore.
In my opinion, 10m has done a lot of damage to a lot of servers. Without the strong incentive to raid 25m, it's very hard to maintain one on a low-pop server. Encounters these days are generally unforgiving and finding quality players for 25m is a nearly impossible challenge on many low-pop servers. I know through the experience I went through on Firetree that a lot of players that are veterans to a low-pop server will concede to raiding a 10m with the remaining group of their friends simply because filling a 25m is too much frustration. Not only does this kill 25m raiding, but the community on these servers as well, as these guilds will become introverted with 10 players that don't change, inciting no need for the group to reach out to the server community for support. Also, in my experience rivalry is much less frequent/intense between 10m guilds, again hurting a servers community.
TL;DR: Yes, merge dying servers! And also, IMO 10m plays a huge part in killing servers.