I would think that as the old system is removed, more phase one racks will be added, rather than moving to phase 2. I'm sure there is no lack of 851/ 700 Mhz frequencies.
The operational drawback of having a P2 system that is heavily loaded, is if you have to take a channel out of service due to a failure, or for interference reasons, you are removing two communications paths and not just one. I would think from a technical standpoint, this is the way an agency would want to grow, and then move to P2 when it's absolutely necessary.
Also, if you plan to host phase one users on your P2 system for hardware compatibility or interop reasons, your P2 TDMA channel reverts to a traditional phase one system, and you are now minus a talk path.
But not all system decisions are made by engineers. You can look south to Riverside County for that example in action.