Last time content was too hard was classic Naxx with everything being extremely overtuned
(or pre-nerf Archimonde, but this got fixed uber fast) I'm by no means hardcore or a good player, but I've cleared mythic HFC a few months ago and don't find it extremely hard. Also, if Mythic is too hard for you - play heroic or normal. You have options, they were made especially for this. And as you've said yourself - they are practically the same. I still don't know what you expect? 10m raids only? Easier raids? 2 size options? Heroic and normal are flexible.
I've been playing since Vanilla, and honestly comparing pre-WLK raid quality to what we have now makes no sense whatsoever. WLK raiding was great and we had a lot to do, but well... 4 raids consisted of 1-2 rooms, 3 of which had 1 boss, 1 was rehashed. Leaves us with 2 proper, new raids...
I'd rather not have more TotES/EoE-like raids which are just annoying and break continuity during the raid, requiring you to change locations super fast. ToC was considered a bad raid even in WLK, mechanics were mostly fun and engaging, but... 1 room and a cave? We could at least have gotten few different areas with natural obstacles or so...
So yes, all in all, I prefer 4 fresh, long raids, than 6 with 3 being 1 room and 1 rehashed. I most of all liked the TBC raids' model, minus Gruul and Magtheridon, especially the latter, but this isn't coming back, because it broke the rankings and a lot of players whined about it back in the day.
I never said Mythic is new content, but the differences change the fight enough for it to be fresh much longer