Scarcity is the new trend I'm afraid. Can it be done better and more generous (ie no rotation), of course. Loot boxes are so much worse to me. THAT is predatory, getting people to purchase something for. 1/500 chance of something they want. But like you admitted, it doesn't effect gameplay.
I'm not arguing the system in Gears 5 is great, I just took issue with your overall statement about the, 'it's just cosmetic' argument. We can throw around anecdotal stories and use terms like 'many people' but that doesn't change the fact the people buying this stuff are in the vast minority. It's just a fact that most people don't give a shit and aren't the type of people you describe whose experience is ruined seeing someone else with a gold gun skin they may have wanted. That's why things are priced the way they are and lootboxes and rotation or seasonal items exist.
It all comes down to the fact that we all value things differently. A gamer whose hard pressed for cash at times may balk at a 10 dollar skin, but to a well paid or seasoned gamer who only plays a few things a year that can only squeeze in so much playtime a week, the thought of throwing around what amounts to a movie ticket for something cool, or a jump potion in an mmo to save themselves 40 hours, is nothing. That's worth it and improved their experience. I would say they are probably more of them than there are the people you describe, who get pissed seeing some cosmetic they didn't want to pay for/unavailable yet. Whales keep alot of the industry afloat and I say let them for the benefit of the rest of us.
And again, saying that doesn't mean I'm suddenly 100% in support of every games implementation.