That’d be great if M0 was representative in any way of what M+ is like, but it’s not. M0 is just a version of heroic that ate its wheaties. As such, one has to learn through the key system, which as you pointed out is pretty painful since your key gets downgraded.
So either there has to be proper space for learning in the M+ system or M0 should be adjusted to better represent M+ (nerfed versions of affixes, etc).