If you lose a game you should lose SP

If you lose a game you should lose SP. There is currently no skill difference between rank 9 and rank 30 because people can just accumulate SP without any retracement

When you know in a game that you will gain something, you will continue. However, If there is a chance to loose and you also will loose SP, then you won't have the desire to struggle. probably you will leave. So the game will became boaring.
This is my thought.

SP has never been a good indicator of skill. The suggestion you offer already exists... we call it ELO. (And it too is not a great indicator of skill, but it has more merit than SP.)