SF already has their QB though, there really isnt any reason to keep Lance around, especially if they would have to pay him as a 1st round pick. The Cowboys viewing him as a 2nd round prospect was probably pretty accurate in hindsight, it should give the Cowboys some credibility in their ability to evaluate talent.
I think the big issue for SF to keep Lance, and honestly even the issue with the Cowboys making him the backup is that you really have to run a different offense with him in the game. Purdy to Lance and even Dak to Lance is going to require an altered scheme, probably more RPO heavy, where as Rush is likely to run a similar offense, just an abbreviated version of it as his arm talent is limited.
I think his evaluation from the Cowboys is still incomplete, and itll be interesting to see if you get some whispers of him impressing in practice after it was a very quite year after he was acquired last season.