Six players per team isnt quite the top half, especially when a lot of teams didnt put their starting AHL goalie on the taxi squad. And replacing them with a bunch of 19 yo first and second round picks probably rebalanced that scale somewhat. It would have been a waste of a year of Tomasinos life had he been forced to play junior this year. Its grossly unfair to the kids who are talented enough to play against men, for money, to tell them, "tough shit go hang out with the kids, and oh by the way do it for free."
Most NHL teams did put their 3rd best goalie on their taxi squad. And most teams didn't just use the same 6 players on their Taxi Squad the whole year, they swapped them around and there were often Covid situations keeping players in transit and in limbo, and some players opted to stay in Europe as well. If it wasn't precisely mathematically 50% of players out of the AHL at any given moment just on numbers, it was certainly >50% on weighted average given that it was the best ones who were usually out. If you take the top 200 players out of the league, whether that's exactly "half" or not, it's still more than half the talent. Anyway...
The main problem with the age rule is that without it NHL teams would abuse it to the overall detriment of player development on the whole. Yes, there have always been a very small number of players who are a little too good for junior and not quite good enough for the NHL. That very tiny number of players, which may include Tomasino, are not being hurt by playing an extra year of junior. But the problem has always been that if you did away with that rule, then you would fundamentally change the nature of the AHL, as teams would then bring in practically ALL of their signed junior players, and you'd have the double whammy of diluting both the junior leagues AND the AHL at the same time.
No system is perfect, but the current one is less damaging than that would be. My ideal has generally been that each team could exercise 1 exception per year. Although even then, there aren't 30 players in that category in any given year, there are usually less than a dozen. But NHL teams would all still use their 1 exception if they had it. So then you are maybe hurting 20 players and all the junior teams they could instead be playing for, all for the sake of the 10 or so players who would perhaps benefit from the early AHL start. So at the end of the day, I think the current system is just fine. It wouldn't have harmed Tomasino to score 120 pts and play for the Memorial Cup this year in Oshawa, lots of highly successful NHL players have dominated junior without having it set them back.