No, the current system supports players leaving a gym for a few hours longer to other players better. With the withdrawl option, players would start to designate gyms to specific colors, like “The gym at the Old Church is designated yellow and should always remain yellow, everybody else must be kicked out!” While, with the current system, teams have to rely on other teams, to kick them out (for the coins), and to simply let defenders lose CP over time so it can be reliably captured, even by a relativy low level player.
Reliance on other teams, and answering that same reliance other teams have on you as well, improves community.
As for the 50 coin limit: Say, Niantic decides to increase the coin limit to 100 a day. How long do you think it will take before people start to wonder whether it could be increased to 200 a day? More is nice, but there’s always more than more as well.