You have some good points. The entire thing with Mills being re-signed is obviously unknown. I think Spurs want him back, but they will have to look hard at the cost, together with his performance this season, and the rest of the personnel they project for next season and plans for Tony.
I am not ready to be all #Nostradamus about it, because it's unknown how Murray's development is looking as well, but if Lapro looks serviceable he's a good pickup for this season, regardless of what happens with everyone else. The memory of Ray getting cut bc he wasn't developing his playmaking quickly enough is still fresh in my mind and Murray is very young and raw in PG skills specially. With the very young draftees all I have heard from all of them is that it's a process (Cojo, Anderson and Murray stating this while clearly in developing stages). Spurs are more realistic than the average Joe fan like us and like to bring very young draft picks along with a development schedule that is realistic.
So although Murray is the higher upside/potential guy, Spurs need someone who is steadier and can execute the offense if needed this season, even if it's a lower ceiling player, but who could be more reliable right away and if Murray beats him out, it's a good sign for the youngster, not a bad thing.