Most likely the gaps are so high to keep things going for a longer period of time. Once people have unlocked everything and reached the cap with ranks as well, it's then when they start to look for something else. If they can't keep them selves entertained after that boredom might follow soon. Reaching for all the weapons and the highest rank is a good incentive to keep playing.
If the devs are smart they make sure they publish something new once a large part of the participents have reached their cap.
I'm getting a tad bit nervous now tho, knowing that BF2 won't run on my system I'm also worried that CoD will do the same to me
And currently I don't see a gap in my cap to buy a better gfx card.oh well. i can always play day of defeat, or try out for Joint Operations, or ... reinstall blackhawk down and team sabre and blow stuff up for shit and giggles.
Tried out my medic, sniper, and anti-tank soldiers. Didn't realize you can use externals when climbing ladders. Oh, great, now I can see the guy who shoots me when climbing high oil tanks. 
Had some success in a tank, but still can't fly the helicopter, unless crashing head first into the ground is the object. 
If you need a sniper or a medic, i´m your man 