> That space doesn't usually fill up completely, so you have a lot more leeway in boarding.

Maybe the airlines I fly are just the worst examples, but I frequently fly business/first both domestic and international and the overheads are always full, usually due in part to passengers not in that cabin.

What airlines do you fly? Delta is pretty aggressive about keeping economy baggage out of business even on domestics. I can’t imagine anyone even trying on an international. More to the point, I rarely have overhead problems on international flights (actually, I don’t think it has ever been a problem). Only domestic legs are problematic, mainly smaller 737s and 320s.

United predominantly.