ICS has a ways to go before it can/will replace IHO CM7 and many are not ready or capable of working with it yet. It takes a pretty beefy system to do it in a reasonable amount of time and uses a lot of drive space. Besides that, there is still a LOT of minor work to be done and it's the minor stuff that can take the longest. I doubt it will take as long as GB did to reach a very good, stable state, but it will take some time. Development at the moment is very fast, but it will slow down pretty soon, it's going quick because of a major breakthrough, it won't stay that way.
As for replacing IHO, I wouldn't hold my breath.
The storage space is a big issue many will not be able to deal with. It may never become the main rom on the OV for this reason alone.
They have have had ICS working on the Evo to some degree for the last 2 months. For every ICS rom available, there is probably 2 MIUI, 5 Sense, and 5 CM7 roms, that doesn't even count the older roms (yes, there is a TON of roms for it).