Solo leveling is not broken by any means, it was just too good before. As you said, if you can just solo leves and get 400k SP why would anyone bother joining a party? Mobs are tougher, you lose durability upon death, and now XP chains. It is fairly obvious that the direction this game is going is towards party play - as it should be. This is an MMORPG. If you really still want to solo, levequests are still there and still award nice xp. Just don't expect to blow through mobs 10 levels above you in a matter of seconds anymore.

The one and only thing that needs to really change is the search function.