I understand both sides: Staying at level 15 is a choice, the entire game is a plethra of choices and options. Although the players that stay at level 15 get to quickly build their attack and defense, it comes at a cost. These players will get passed by other players that actually want to get their dks. Therefore the options are clear, make a name for yourself by getting dks or make a name for yourself by getting pvps. Also, as stated before by someone, some players stay at level 15 for various options like financing a castle or cityhouse, building their attack and defense (pvp), or controlling a battle arena, etc... When we take away options from players we are losing the core of the game.
In contrast staying at level 15 has also ruined the game in some aspects. The meaning and purpose of the game is to get as many dks as possible. Having people stay at level 15 and be able to kill others that have more dks, doesn't make any sense and it makes the game unbalanced. After reading Cyrus and Shukaku's comments I agree with them to a certain degree. However, I think it's best to not stop people that want to stay at level 15, but to instead have a ceiling. By this I mean that players that stay at level 15 will eventually reach a ceiling, where the player according to their dks and rank (acad.student, genin, chuunin, etc..) will not be able to get any stronger or build their attack and defense. What this does is still give those players the option to continue to rack up pvps, but there is a limit now and it encourages them to eventually get their dk and once again if they want...they can stay at level 15 again and be stronger than they were previously. It also gives the people that we're killed by them a chance to get a measure of revenge before the players get back to level 15 and kill more people again. It's a cycle, but the ceiling of power raises with each dk.
