1.5 allies seems conservative to you? Not just in melee, but attacking a creature adjacent to you, correct? With all the talk of the dominance of ranged attacks and all the casters with decent damage cantrips, I am surprised to hear that so many characters would be in such close quarters melee. I wonder you are running more dungeon crawls or urban games.
My experience 5e is mostly TOTM, outdoors or in large caverns or ruins, and with 5 players or less. I prefer sandboxy games, so enemies range from tons of easy to defeat, to ones so large that they cannot be killed conventionally, and the enemies don't follow any rules of "Mook Chivalry" or similar. Very few enemies will group up, many more will disperse and try to circle around to the ranged attackers (and casters). IME, a lot of players really dig the hit and run, ambushes, and alternative tactics that have been easier and more fun in this system. The few players who play melee characters are much more likely to spread out to protect the ranged ones, or for the rogues and monks; jump all over the map. The barbarian is very likely to charge into melee, often to attack the back ranks, but he isn't likely to wait for the slower fighter. Maybe the fighter will catch up, and he will have 1 ally in melee with an adjacent enemy, but not for most of the fight, and 2 would be exceedingly rare.
I honestly don't think I have seen an average of 1 ally in melee adjacent for the barbarian. Now, I have no idea how uncommon this is in the general community, it is just what I have seen.