The problem with Murray isn't just his shot (though it will obviously improve some, virtually every big PG's tops out as below average shooter), it's the lack of a single foundational offensive attribute (speed, quickness) or skill (ball handling, finishing, passing, vision) to build off of. He might physically look and sound the part of a foundational player, but if he wasn't a Spur, I question how many would feel confident in his becoming one.
He also times poorly with the majority of the roster, specifically Aldridge. Even if he becomes the star many seem to think, Aldridge won't be at that point, leaving them in the same position they're in now.
But the only scenario where I don't think Murray is the starter, is if Leonard is traded and it's to the 76ers, with Fultz being the centerpiece of the package
Mills, Murray and Parker, will more than likely comprise 75% of the guard rotation. Murray and Parker can't play together offensively, while Mills and Parker can't play together defensively. So by default, that means Mills and Murray have to be paired together.
Murray and Parker are useless off ball and a dynamic guard would be wasted playing off ball next to inferior offensive players. In other words, they don't need to bring the latter in to supplement those two, but upgrade them (which isn't happening).