While I don't support the idea of replacing the LMGs with that concept in ordinary units because of couple of reasons I find that concept very useful in CQB situations. I would like to arm counter-terrorism units with this and increase the mobility of the team while at the same time I keep the firepower potential high.
Good luck shooting comfortably while you keep your silhouette low in prone position with this. As I said above I don't support that concept for every scenario. I would like to see that concept in a tactical response units which require lighter platforms that can still provide firepower superiority and also provides the capability of fast reloads in situations where every second can be of critical importance.
50 or 60 rounds could be just a limited tool for special roles.
But these magazines no doubt proved themselves more useful than drums.
Drums, helicals and other new forms of mags are barely used due to multiple reasons.