Exactly this. A distro is a software distribution. It’s usually not a smorgasbord of whatever you want (unless it’s Arch or maybe Gentoo). Decisions are made so the whole system works together properly: a package manager, an init system, security, networking, shell, GUI. These pieces are all distributed by the makers of that distribution, and configured and designed to work together.
You should only think about ripping out and replacing these components if you’re in the “build it yourself” scenario (i.e., Arch, Gentoo, or OM Slim). If you’re in that scenario, you bring with it the expectation that you’re the “distributor,” and any problems you encounter you will have to fix yourself.
The only takeaway I can think of for OpenMandriva is maybe instead of saying Slim is for people who want to choose their own programs, saying that Slim is a minimal system for those who want to build their environment from scratch, and a minimal Plasma desktop is included only for convenience. It seems like there have been a few people recently who started with Slim because they didn’t understand what it was for.