Technically, what's to stop Nexstar from operating WPIX? Mission will hold the license and programming, and that's the extent of FCC jurisdiction. If there's some random UHF station that Nexstar wants to buy for "ATSC testing" or for some other purpose, then they can get that for half-credit. Where does Nexstar stand on their cap at this point?
This is a one-off transaction, so even with how the Sinclair-Tribune deal fizzled, as long as it's Mission calling the shots and buying at fair market value, there shouldn't be a problem.
Sinclair failed miserably with "getting" WGN by finding one of their buddies to sell to, at a significant discount.
At stake is WPIX's carriage on YTTV (which was a group deal with Scripps for the other Tribune castoffs). This would make it the lone CW affiliate carried by a Mission or Nexstar owned station. Nexstar wasted no time in pulling WMYT from the Charlotte market (since FOX had it carried before). Hopefully a larger deal emerges that includes all of their CW stations and WGN eventually. This is a major hole in many markets.
The timing on this was likely deliberate to see the end result of the FCC's decisions on Sinclair.