I wouldn't say that is entirely true at all. It isn't like a sbmm system only matches you against people your exact skill level. It is typically a range. And so you face people around your level, allowing you to improve by facing those at or slightly above your skill level. You can still improve by doing this, it is more so up to the individual if they actually want to. It is the exact same way that people who play games that use a tier system end up going up in tiers, or ranks. They face people around the same level, they get better, and then they eventually move up a tier when that tier would be more appropriate for them. It's pretty much the basics of how any good sbmm system works. Halo did just fine with it for years in their ranked playlists.