Mags can be changed pretty fast, and these rifles are not used by just one dude fighting on his own. There will likely be at least a squad of other dudes close by supporting each other, so mag changes aren't so bad.
You have to load new belts with a belt-fed as well. They last longer, but the loading process is more involved than slapping in a new mag.
The idea is to have a variety of arms supporting each other and making up for the weaknesses of the individual systems.