If you'll indulge a combat fantasy for a moment...
If we're in a guerrilla/red dawn type environment, where we are defending our territory against a numerically and technologically superior invaders, I see two general types of engagement with that enemy:
1) long range scout (potentially sniper, but that gets into another thread)
2) ambush/raids
#1 is not looking for trouble necessarily, but may engage targets of opportunity. If you're just one dude, getting some distance between you and return fire seems desirable.
#2 requires the attackers to get in and out fast, and maximizing damage and casualties as quickly. Think of ambushing a convoy or raiding some supply cache.
This was true a century ago, and frankly I don't see the fundamentals changing a whole lot even with aerial drones and all the fancy tech today.
The conclusion I draw from this, is that the .30 cal battle rifle is suboptimal for either above. In a way its a compromise between precision bolt and fighting carbine. For the raid, I want light and fast. A physically light weight load out, but also something with fast repeat shots. For the scout/sniper, you may only squeeze off a few rounds total, so accuracy and reliability of hits is more important.