you can easily Upgrade to +11 +12 with a few days of farming. This game is way to easy. I don't know how you farm or how much you farm but you get your end gear to fast and +11 +12 is enough to do everything. If you play active you need only a few weeks to win budos and do all pve content (only gear skill is another story).
Talk for yourself. I recently used 70kk and ended up with +7- +10 set. Yes I do use white stones and yes I do use advanced ones, so not everyone can get +11 in a few days. So what is your suggestion? You think making item destroyed would make it harder? You would just need more white stones at early levels and that's it. The only reason retail didn't have many +15 is because it was more p2w and you couldn't reach that without cashing. If bots ruin it then they will exist no matter what you do about the upgrade system. Remove +3 from advanced (can't believe i'm saying this with worst upgrades) and they won't jump to +15 from +12. If you got better ideas than broken cards destroying your best gear then please share it with us.