I agree - it would simplify everything whilst at the same time giving the entire server a universal touchstone to measure progression.
Have one or two entry level raids set at 8 man (with lower level reward epics) to get people used to larger group sizes, and then upper tiers kick off at 16 man raids with progressively harder group mechanics in each tiered encounter.
Basically how WoW was pre WotLK.
Karazhan > Gruul > Magtheridon's Lair > SSC > TK > MH > BT > SW was the best progression flow of any MMO I have played and the difficulty tuning was spot on.
The problem with your progression chart is that not all of those instances came into the game at the same time, so comparing all of BC to the first month or so of a new MMO isn't quite fair. Just consider the fact that in the future there will be further operations that you can compare on a similar level to WoW. Don't try and compare it a month after the game came out. Consider the fact that it was basically trying to find 40 people for Molten Core right after the game came out and even if you are talking about BC, the raiding still came out gradually.