To broaden the discussion outside of the guys who are "just all-american" i looked at the sophomores who placed in the top 50-100 in 2010 and how they did the next two years (sorry if the formatting gets screwy)
david forrester 53 16 62
bill kogel 54 29 51
matt mildenhall 57 74 108
wesley rickman 59 234 77
lars erik malde 62 133 115
matthew bayley 63 43 DNR
andrew bayer 67 39 DNR
zachary mayhew 78 64 13
eric fernandez 79 38 83
sean keveren 82 DNR DNR
jeremy rae 93 105 DNR
reed connor 95 36 12
ross millington 96 DNR DNR
kevin williams 99 53 78
It's interesting that the results are actually better for guys their junior years than senior years. Nearly half (6 of 14) of the guys made jumps in to the top 50, with another guy going from 99th to 53rd, and improved 37 spots on average. You've got three guys that ran slightly better or slightly worse, two guys that ran way worse, and two guys that didn't run (which could mean a variety of things).
Their senior years, you've got 2 guys that have made big jumps to the top 15, 5 guys that finished in the same ballpark as their sophomore years, 2 guys that did a chunk worse, and 5 guys that didn't run. I believe Andrew Bayer redshirted xc, but if he ran there's a good chance he would have been in that group that made a jump to the top 15.
I think the main conclusion here is that it is definitely hard to consistently improve your place at the national meet, but also that lots of guys at this level do see improvement beyond their sophomore years. The fact that half the guys made solid improvements from sophomore to junior years shows that it doesn't take anything exceptional to improve and that coaches can certainly expect their guys to be able to do it, but also that improvement doesn't come automatically. The inconsistency with senior year results suggests that over the long term, guys have to be doing a lot of things right to keep improving. Some guys will be able to make a big jump, while a lot of guys will fall or end up around where they were sophomore years.