Your logic would hold for people that are already clearing to Deathwing regularly (let's assume on normal), and assuming they're not moving to heroic mode but staying on normal. If no one improves their gear week to week and no one improves their skill, then killing Deathwing faster after each nerf would be a false illusion of "getting stronger". But for those who have not progressed to Deathwing, then killing any bosses between Morchok and Deathwing is "progression" by definition, regardless of whether the bosses are nerfed. That is not a false illusion, that actually IS progression.

The last numbers I read, from the end of December, are that about 35% of the level 85 player base has completed Raid Finder, as opposed to 200,000 out of 7-10 MILLION subscribers (whatever the number is) that have cleared Dragon Soul on 10 man mode.

As this article says, and as many people in the comments have said, the nerf is OPTIONAL. You can turn it off and keep killing Deathwing at the same difficulty level as pre-nerf. You can move on to heroic mode and turn the nerf off so that your heroic mode is the same as pre-nerf. It's up to you as a group. If your group can't agree on whether to nerf it or not, that's not Blizzard's problem.