There are two reasons why I disagree with your idea.
1. If you train a level by misclick (5-10 defence), you will need 1-2 lamps to reset the xp. Lamps cost 30-50k ea. So don`t tell me about lamps being expensive or that nobody has the money to buy them.
2. There is one item that I have in mind that can be abused with locking xp. What happens when a maxed 60 att pure trains 70 attack and locks the xp? He will wear an abbysal whip (more accurate and +2 higher max than dragon scimi) and will stay 86-88 combat forever while using an overpowered weapon in his combat range.
1. Okay, yeah, that's right.
2. Think of it again, you will see more 86-88 comat levels there, not just only one, they would go against each other with an overpowered weapon + people with abby whip + more risk won't take on some random dragon scimi dudes