1 Main Server and 1 Seasonal Server should be the standart of an MMO in these days, because the most fun part of an MMO is first a few months where you competing for leveling and gearing with others. A seasonal server(3-6 month) can be fast paced and where you can reach at least half of the end game with casual play and reach %75 near to end game at the end of the server then all seasonal server character will be transfered to main server. Then a new seasonal server starts.
This way at least every 3-6 month new players can come to game instead of waiting for all servers die slowly.
New patches, new contents cant successfully bring new players or quitted players back to game. Especially like this game where the gear gap is so hard-capped.
The devs should already learned this, MMO Players always likes to play on new servers/new starts, because competition is high and exciting there.