What is there about the eye test that suggests to you that Allen is going to be starting quality? I'm willing to completely overlook his defensive liabilities as most rookies suck on defense in their 1st year, but you watch him play and he's a guy who is a shooter but hasn't really shot well (and he wants to jack up shots if given the opportunity), has a lot of trouble finishing at the rim due to his lack of length, doesn't really have the skills to be able to run an offense at PG yet and doesn't have some explosive 1st step to get by defenders on the perimeter consistently in the NBA.
I look at the stats for him because what Grayson Allen projects to be in a best case scenario is a lethal shooter who is also a tough nosed defender on the other end, and there's nothing really to suggest that he will reach that best case scenario unlike someone like Mitchell who clearly showed star talent from summer league all the way to his inconsistent playing time before he cracked the rotation and eventually became the ROTY contender Mitchell.
I don't care for G League stats because plenty of rotational bench pieces or 3rd stringers in the NBA can or have gone down to the G League and dominate (which is what you'd hope for if they're on a NBA roster), and it's good that Allen at least can show that he does belong in the NBA. Using G League or college stats where the level of competition is significantly weaker and prioritising those over his NBA stats just because he gets more minutes is stupid though, and right now in Allen's young NBA career he has shown that he can't really play to the level of rotation NBA players (which is why he's buried on the bench), and there's nothing about him both from an eye test perspective or from a statistical perspective to suggest that he will become a starting caliber NBA player. You could watch Donovan Mitchell and see him have a stretch of play or do something which made you think "He's going to be special", but there's nothing about Allen's game that makes me think that and there's not even much there to suggest he could be a decent starting player.
It's not really a knock on him as he is a late 1st, and I'll keep repeating that him developing into a quality rotational bench piece is still good value for a late 1st. But at the same time there's pretty clear limitations on what he could be going forward and I don't think it's unfair to use his awful shooting %s in comparison to other 4 year college vets who came into the NBA and took a few years to develop but at least showed something % wise in their rookie seasons to suggest that they had a chance to develop, and overall he's just been underwhelming as 4 year college vets who are taken in the 1st round are expected to be able to contribute and Allen simply hasn't outside of a couple of stretches over the season. And before you tell me it's because he isn't getting consistent playing time, if he was actually capable of contributing to the team consistently he would get the minutes, as he's had opportunities to prove himself with injuries and the like but he just wasn't that good. Royce was in a similar spot earlier on in the season but has since re-established himself in the rotation as a key rotational piece from the same opportunity.
If there's something about Allen's game that convinces you from an eye test standpoint that he's going to be a good starting caliber player then I would love a link or something, but I'm not just boxscore watching like you're projecting. I think most people here would agree that Allen hasn't shown anywhere near the amount of intrigue/talent/whatever from an eye test standpoint compared to other guys on the team in their rookie seasons like Mitchell did early, what Royce showed in flashes, or even what Exum showed years back in his rookie season.