Are you sure you're playing the same game as I do?
You say, that on your previous server, all people were playing different classes, so do people here. If you see a dps player play only one class over and over again, I'd say it's not balancing's fault, but caused by the inadaptivity of that player to the endgame (or let's better call it laziness). I see a lot of players stick to only one class, that they were told to be "the strongest" but if you really analyze the gameplay properly, you'll see that there is not the one perfect solution for everything. For example, Mage/Warrior is a decent class with tons of magical damage and decent matt for burst. In theory, this class comes close to be the "perfect one for every content", but actually it isn't the best in slot class for everything at all. Even a Mage/Scout can easily out-damage it in instances with higher defences, since pet classes (almost) always beat non-pet classes 
If you try to play the very "best in slot" class in every single instance, you simply have to play more than 4 classes at the minimum. If you even wanna run best in slot classes in every content (to include endgame minigames (such as AoD), nostalgia instances (such as Hoto, Kalin, IDK), event instances (such as Atlas Defence) or even grinding instances (such as IBP or souls) as well), you end up at even more.
And if you additionally wanna actively adapt to your raid setup, in order to support your friends, there are more options available again. You can simply play every single magical damage dealer and find a raid setup that is best for it. COA is not about balancing single classes against each other, it's about how to properly build raids and double your overall damage just by class interactions and synergies...
To be honest, a lot of people seem not to understand how to properly build decent raids or optimized groups. They may run with the same classes over and over again, this works for sure, but isn't optimal by far. There are so many possibilities to improve the overall dps, damage reduction, healing, burst, CC, etc, by so many different ways, it's complex enough to start studies on 
Example: You have 4 mdps players. All of them could play Mage/Warrior and every single one would deal decent damage. But what if one goes for Mage/Rogue, one goes for Warlock/Rogue, one for Warlock/Champ and one for druid/rogue? Then you got huge debuffs for dark damage, gift of the baron = dps, damage reduction, whisper = matt, and every single class deals more than M/W would have dealt without such synergy. In this example, your leather players could go for Rogue/Warlock, in order to get a piece of that huge dark damage support. Also, the druid/rogue brings druid buffs, so the healer could play something different, like warden/priest for even more support for the group, only by picking the right choices. And all of the classes deal equal damage 😱 That's how we play 6 man content and that's how you can run abbey within 20 minutes easily... instead of like an hour or more 🥵