I guess that inspection/maintenance is why some turf fields last longer and stay nicer than others?
I've never seen a side by side comparison for turf vs grass in terms of cost over the 10-15 years that a field can last. I assume grass is cheaper, but turf is WAY less work and provides a consistent surface for soccer, football, etc.
I have read the studies coming out about turf and injuries- most are soccer/knee injury related or head injury in all sports. I don't know that there's a study out that compares football knee injuries on turf and on grass.
I can tell you that the footwear has changed drastically. Today's cleats are almost all designed to be on turf and they have more spikes, nubs, points, etc than anything I ever wore. I wore screw-ins on turf/grass, never had an issue with traction but I think the increased surface area that modern cleats have might have something to do with the amount of injuries.
Tackling is also taught differently. To avoid head injuries, football has gone more to the Rugby style tackle, but that also promotes the "gator roll"- another reason why lower body injuries have probably increased.