I can't offer any insight into the relative worthiness of the iMac screen but I can tell you that monitors (even Eizo) do not last forever. I recently replaced my previous Eizo with a new one after it had exceeded it's expected life hours and would no longer calibrate correctly. My own personal preference is to keep the monitor and CPU separate in order to allow for the flexibility to change either of them. For me, this is a business expense. I completely understand the attraction of the iMac and have seen it used to great effect.
Best,
Tim