However as soon as people get stuck in the progress, they will stop getting gear upgrades, but will keep accumulating VP to upgrade existing gear. They also have a chance at warforged gear, so even if they are stuck, there are potential upgrades from raid drops.
There are basically two ways of progressing when your group is stuck at a boss:
1) Improve your gear
2) Improve your gameplay
During normal progression, both tend to improve until the boss dies.
The reason nerfs become necessary are when you are faced with a group who already:
1) has all the gear they can get from bosses prior to the one they are stuck at (ie they are no longer able to improve their gear without defeating the boss)
2) have hit their skill cap (ie they simply aren't able to get any better at the game)
Effectively said boss has become too hard for that group in that gear to ever beat (at least not before they give up), and without beating the boss, their gear can never improve. So they become stuck, unable to progress, until they eventually give up.
Now, with item upgrades and warforged, groups stuck on a boss will still see gear improvements for a long time even if they are no longer getting any "new" gear to help in their progression. For most groups who may be stuggling on a boss in spite of having the best items from the bosses before it, an extra 8 ilevels will be a sufficient boost to make the difference.