I actually do think Greenwood has been underused this season. He's got 3 times as many goals as Dan James in half the starts. I know he's young but 5 league starts? Not enough imo, especially in a team that struggles to score. The things with Ighalo staying and Sancho seemingly on the way, I don't see a huge jump in opportunities for him in the first team next season.
I see what you're saying, but I think easing him in cautiously is precisely the reason he's been so successful. If Ole had just given in to the hype after those first few appearances, he'd have scored in a few more starts, the papers would have absolutely lost their minds over him, the pressure would have mounted, and a few goals without a game would have seen a massive backlash. 'Overrated', 'beginner's luck' etc. Being rotated out of the team would have been seen as being dropped, and potentially really hurt his confidence.
Ole played it perfectly, dropping him in here and there to score goals and build his confidence and his teammates' trust, but then lifting him out again to prevent a build-up of hype and pressure, and also to allow him to continue to develop physically without too much risk of an injury, which can be a disaster at this age.
Now he is settled, he is a stable part of the team, the press knows about him so if he starts banging them in there won't be such a hullabaloo about it. Now is the time to make him a regular. But we've got here the right way, by taking it slow.