Common sense? Loot boxes that gives you an advantage is not ok.
I really want to be snarky here because I think it's funny, but I'm not gonna. I'm gonna assume you're arguing in good faith, so...
1) the argument you are making is generally the PVP related argument, which I am fine with. It is, essentially, "it allows rich players to spend real money gain competitive advantages over poorer players, which sucks." There are lots of reasons this sucks, like poorer players generally not having the same amount of leisure time to grind and be on an even playing field (it's a lot easier to work a 9-5 office job and blow a hundred bucks on loot boxes like it's nothing than it is to work 50-60 hours a week like I used to for less than minimum age just tryin to survive), stuff like that. It's shit and it's inherently unfair. I agree.
2) the problem with this argument is that you haven't taken into account the games in the OP, which are not competitive games. Consider, for instance, Forza 7. The loot boxes in Forza 7 do not offer competitive advantages, nor are they cosmetic only. Many of them drop cosmetics, but the main thing they drop is mods. Mods are more like random challenges; they basically give an XP or credit bonus to players
3) there are plenty of other games that implement non-cosmetic loot boxes in interesting ways. Consider Warframe's relics, which are loot boxes with specific drop rates for individual items within them. Players can get relics through gameplay, then run missions to open the relics, then get item drops in them. Opening a relic also gives you voice traces, which allows you to adjust the drop chances of items in the relics. What are these items? Prime items. You can just spend $80-140 to buy them all, or you can farm for a few weeks. I have all Prime warframes in the entire game thanks largely to relics, or platinum I got from selling Prime parts I got from relics I didn't need. Warframe is a cooperative game, so they don't give you an unfair advantage. Anyone can get prime stuff through gameplay.
Basically, the OP's argument is a pile of steaming shit, presuming that a game mechanic OP doesn't like is inherently bad, and only good when it's done the way the OP likes it. The OP has not thought this through, or, if OP has, the OP has not demonstrated as such in the argument.
What? People can decide for themselves where they want to draw the line in the sand.
OP said loot boxes were only okay when they were cosmetic.