why not? wouldn't it be viable? I mean there are tons of games that are not crossplay, inf fact 99% of them, that receive all kind of updates without problems, even from small devs, so i am sure the money maker that is minecraft could afford it.
It is easy. MS wants unified platformdespite knowing is dealing with different online services, and can do what they see fit, but it is a business and strategic decision and use their strong position owning an important IP and putting sony in a difficult stance.
Why should sony allow it, wouldn't it defeat the purpose of a proprietary online service?
You answered your own question. MS wants a unified platform. Minecraft players on all platforms would greatly benefit from this unified platform- even Sony players.
Why shouldn't Sony allow it? How does it defeat the purpose? PS4 players would still need PS+ to play the game online. The free xbl account is just so minecraft players on all networks can use minecrafts social features together (friends list, messaging, private realms etc) and so that their purchases can follow them.
By not allowing it Sony is essentially ensuring PS4 start looking at other versions and stop buying Minecraft DLC on PS4. They are weakening their proprietary platform.