... maybe with a slightly lower item level and embrace the fact that it can't be tuned quite as hard as 20m?
It seems to me that besides other problems (garrison prisons etc) the new 20 man mythic raids are a nail in the coffin of many old small heroic (now mythic) guilds who like to stay family&friends size instead of recruiting 10+ more people/merge with 1-2 other guilds.
Some people say: "transition to 20m mythic, it's easy" and from their perspective that may be true - however it ignores the simple reality that a lot of players can't do that or don't want to do that for one reason or another. Many successful ex-10H raiders of past xpacs find themselves out of raid content this time around in an expansion where there is literally nothing else to do besides raiding. Blizzard might have hoped that they'd transition into 20m mythic, but you can't ignore the simple reality that this doesn't happen as much as they might have expected. Instead, these players take a break until the next tier or unsub and play another game.
This will only get worse as the expansion continues and will also have ramifications on guilds who managed to transition to 20 man mythic as the pool of competent players dwindles and recruitment gets harder and harder.
In my opinion everybody should be concerned about this development. Yes, even players who successfully transitioned to 20m mythic. The customer base of an MMORPG is an asset in itself. Less raiding players = less people to recruit. And on a higher level: less subs = less money for future content.
Blizzard removed the old ex-H10 raid content and does not offer a suitable alternative for many people. In my opinion the removal of options/content is rarely a good idea.
I don't like PvP for example, that's why I don't play it - but I can see that there are people who do enjoy it. Yes, if Blizzard would get rid of PvP, in the short term that might even have a positive effect on my own experience because offered the alternative "PvE or quit" it might make some more players do PvE content and Blizzard could concentrate on balancing PvE. But it would surely alienate a lot of players who liked the previously offered content, who liked PvP and it would drive them away from the game. Ultimately in my opinion it would do more harm than good, and I think it's the same thing with the removal of the ex-H10 raid content.
It would be great if 10 + 10 = 20, but in this case it just doesn't work that way for many players.
Ion Hazzikostas said they like the 20m format and guilds were dying all the time. I think having a single hardest difficulty level with a fixed number of players has its merits - however many small guilds don't care about world firsts and stuff, they'd rather stay small and still have something to do after normal (now heroic) mode.
At least I'd rather do 10m mythic with my small guild and get a few less item levels than saying "gz, we beat heroic. now we can unsub until the next raid tier comes out".
Blizzard said tuning an additional raid difficulty only takes 5% of their dev time, so it surely wouldn't be hard to implement.
It should share a lockout with 20m mythic of course to prevent players from thinking that they have to farm both 10m and 20m like in WotLK.
Looking at the MMOC numbers some weeks ago you can see that about 20% of all players have killed nearly all BRF heroic bosses. Yet only 5% managed to kill the easiest mythic boss. I think it's safe to assume that many(most?) of the people who clear heroic but can't even kill one mythic boss don't manage to do so because they're in a 20m guild which is just too bad, but because they don't have the players to even do mythic raids regularly. I think this raid content hole after heroic for smaller guilds needs to be filled.
I also think that taking an entire raid tier away from 10 man guilds is especially problematic in an expac where there is literally nothing else to do besides raiding and guilds having to deal with players losing interest in the game at a rate never seen before.
I can see where Blizzard was coming from, but I don't think it is working out. I don't blame them for trying something new, but it's important to recognize when your experiment didn't have the desired results and do something about it.
edit: a lot of people are pointing out that you can't achieve equality between mythic 10m and 20m, but that's in fact my point.
If you don't like to call it 10m mythic, let's call it something else then. "epic". "slightly less mythic than mythic, but still pretty tough".
The key features should just be:
1. harder then heroic to give 10m guilds something to do after heroic.
2. shared lockout with mythic, slightly lower item level.
edit²: I'm quite aware that the raid difficulties were renamed in WoD and technically there has never been a "mythic" 10 man raid so far.
MoP WoD LfR -> LfR Flex -> Normal Normal 10/25 -> Heroic Heroic 10/25 -> ??? /Mythic 20
But to keep the title short and avoid confusion with the old pre-WoD nomenclature I've decided to use the new name and put "back" in quotation marks.