They are in the PERFECT position to do this during camp, practices and the regular season. That is how you normally earn minutes. The problem with Monty is he is fiercely loyal to players. For example, many NBA analyst has said that Holiday looked impressive during Suns practices, and he looked good here in there in games. Yet, Monty never really gave him the chance out of loyalty to Payne. He did something similar with Frank the year before. Nobody knows if this kid will pan out or could beat out Payne but Monty seems to limit those opportunities out of loyalty for some players.
Go back and read the forums the past couple years. You will see post after post about Frank the year before and Holiday this past season even though the players ahead of them were playing like complete crap or couldn't stay on the court for stretches.
I saw it with Frank. I was pissed he was cut from the rotation entirely in the 2021 playoffs.
I saw a lot of posters calling for Holiday but don't recall any reporters saying he looked good or anything like that. He looked decent in limited minutes during the season but he very rarely played PG. He was typically out there with Payne, Paul, or Booker and playing off the ball. Holiday isn't a PG. There's a reason Washington gave him away for nothing. I don't fault Monty for sticking with Payne in the playoffs.
Teams don't play 13 deep. Monty was already going 9-10 deep with the starters, Payne, Shamet, Cam J, Craig, and McGee. Ish, Biyombo, Holiday, and Payton rounded out the roster and frankly if I'm coaching in a playoff atmosphere then I'll go back to my starters earlier than going deeper on the bench. I can't think of any coach who goes deeper in the bench in those scenarios.
That's not to say he couldn't make adjustments between games but there's limited time available and size was an issue in both playoff series this year. Going smaller with Holiday wasn't going to help considering he's the same height as Paul and look at how defenses threw him off.