You're missing a crucial point here.
The issue is not with the amount of players we have, it's with the amount of machines and money that can be spent on the server.
As the server is 100% free and none of the players is not paying a dime for even the bandwidth required, let alone the hosting bill, we are stuck on limited capacity.
If there was a way that the players could help out with the bills then we, the players could upgrade the hardware and bandwidth so that we can have a better experience (not talking about money donations, because Blizzard will shut the server down the moment anything viably resembling donations pops up).
Honestly i've thought of many ways to do this, but it's up to the dev team.