I think I have expressed my concerns earlier in this topic, but there really isn't much space available on the South/Southwest side of town large enough to build a new high school campus. That's why I don't see TCU purchasing the Paschal Campus.
I agree with you completely with regards to TCU passing up the Paschal property. That said, there is some land within the Edwards Ranch property that is still empty and might be suitable for new schools. Something in that empty space could work for the planned Tanglewood "reliever" elementary school, but you could fit in a new high school and middle school (to relieve PHS and AHHS, as well as McLean and Monnig) somewhere on the remaining Edwards Ranch land too. Trimble Tech could also become a zoned high school as well.
TCU is going to need more land to expand, so it's just a matter of where you can get that land. One thing that could be done could involve taking commercial properties on the south side of Berry Street and building new mixed-use properties where the bottom floors could accommodate retail, with academic space on the upper floor(s). I believe Texas Wesleyan did something similar when they redeveloped the south side of Rosedale just across from their campus, where certain academic uses were mixed in with other retail. Of course, with this being TCU, a similar redevelopment along Berry would have to be on a much larger scale than what happened with Rosedale and Wesleyan. But you wouldn't have to go much further past Berry Street if this were done right, and you could spare the neighborhoods to the south from TCU's expanding footprint.