It would have been weird cause Moses and Walton were nothing alike. Read
The Breaks of the Game - that Blazer team totally revolved around Walton's passing and decision-making, if they had Moses there instead it would have been a completely different team and wouldn't have been nearly as successful.
And they couldn't keep Moses because they literally couldn't afford him. NBA wasn't very popular yet, didn't make much money at all in the 1970s, and teams often gave away good players who they simply didn't have the money to pay.
The even better what if? The Blazers traded Moses away and got the #1 pick in the 1978 draft in return. They picked Mychal Thompson with that pick. You know who they passed up?
Larry Bird.