Second, it would involve a ton of product testing, and tedious ironing out of rough edges. Every single time an installation or upgrade fails, or runs into a strange error needing obscure forum searches to fix it, that's a problem.
Third, they'd have to seriously cozy up to proprietary software vendors for application support. This one is unfortunately an endless time and money sink, and can't pay off until they have enough users that they don't need to do it anymore. Thankfully F/OSS options provide some limits on this, but not entirely. Many times there are F/OSS options that are usable, but the proprietary ones are better (and worth paying for).
This is just a few thoughts off the top of my head. Its not well researched or thought out, but a starting point for a conversation.
Second, Linux systems have a bit of a jerry-rigged feel to them that needs to be addressed. Things like fd.o and the FHS are intended to help this, but don't go far enough. Compare this to FreeBSD, where every decision about which packages are used in the base install, what the filesystem hierarchy is, etc. is well documented and cohesive. Linux can't go mainstream until, say, a runit-based distro has no references to systemd in its documentation.