Bespoke 10 man content needs adding to the game, not 10 and 20 man raiding of the same instance. Not getting rid of 20 man raiding to make it 10 man.
Blizzard has turned away from trying to be the protecting hand, no longer really trying to stop players who want to push excessively hard from being able to do so. It'd be nice to have more content to turn your hand at.
Unless its a part of their design philosophy going forward (such as losing both 10 and 25 in order to have 20 man raiding) no one is losing content they enjoy, Blizzard spends its time looking for new content to add that players will enjoy. With procedural generation, there's no reason why some new content could not be developed with a new niche that is the challenge 10 man mythic would be.
I much prefer 20 man over 10 man, but I'm not a fan of saying "suck it up you've gotta get used to 20 man" because it doesn't help. I don't oppose 10 man content, if there was 10 man content in the game on the odd weekend here and there no doubt I'd smash through it with some friends as well. I still miss that during the weekends of ICC we'd have a small group from our 25 man raids who would usually go and do late night Ulduar 10/Trial 10 raids.