I'm not sure why some people are not open to the % decrease if the player isn't playing. The whole point is that if a player is fit and available and not being picked, and they are not happy being a bit-part player, they would have the opportunity to move on for first-team football. They don't have to move if they are happy, it just stops situations arising that might stagnate a players career for the sake of a clubs mindlessness.
As for the comment about trying to get dropped, if he was not readily available for selection he is in breach of his contract. As stated before, the whole idea is to stop the players ability to drop himself. If he is choosing not to make himself available through lack of commitment, the club would be able to alleviate those concerns in the season. I realise this might get complicated just writing it...
If not, maybe we look at the release clause as a base value. Play more than a certain percentage, and the value increases two-fold. I.e. Coutinho is worth £65m to the club now, but after 50% of games, his value in his contract doubles to £130m, to protect the club from losing a first-team player.
The problem is that a player could use that to force a transfer and reduce his release fee, like not play well on purpose to get dropped. If a player is being dropped, his team would try to get rid anyway and that would also prevent teams from piling up players for nothing as the player would know that he will not get games if a team has many players.
A release clause will be enough else it would be too complicated, no system will be perfect anyway