I think the reason for that is quite simple: Challenging content and queue system just does not work (in wow.) Whatever gets added to a queue system has to be adjusted, because once you can queue for it, people expect to finish it within a reasonable timeframe.
This got evident quite fast when we stepped over from Heroic dungeons of WOTLK (which was pretty simple, nowhere near as challenging as dungeons from other expansions) to Heroic dungeons of Cataclysm. Blizzard once again tried to add some semblance of difficulty to heroic back then, requiring not only interrupts, but in some occasions hard CC like stuns otherwise the group wiped pretty fast on trashpacks. There was a massive outcry on this, and I even recall hearing that Blizzard later stated in an interview that they had to massively reduce the difficulty in order to bump up the agonizingly low dungeon success percentages.
You can even see it today as well in leveling content: Generally people avoid Cataclysm dungeons like the plague, because even while it was severely nerfed there are still some mechanics that players must respect (more often then not the tanks.) Even to this day barely anyone goes to kill Erunak Stonespeaker in Throne of the Tides, because the boss required both the healer and the tank to be on their toes, since the boss can literally twoshot a tank without active mitigation within the timespan of two seconds. Pandaria dungeons, which was in the same leveling range, was the leveling choice because it was much easier.
Mythic dungeons were originally created as non-queue-able content for premade groups, and it is where Blizzard wants to keep it. If cataclysm is anything to go by there is a set amount of success rate something must match in order for it to stay untouched, which is why people are weary of letting anything get on the queue system, since it means that it will have to be tuned in order for everyone to do it.
You may say that yes, the success change will be low, and that is fine, but you also have to keep in mind that queueable content means it is for everyone: to Johnny who’s level 12 and only plays retribution because the abilities are flashy, to John who may only be able to play with one hand due to health issues, to Jimmy who struggles through 16 hour shifts a day and only has a good hour a day to play. For them queueable content means that it isn’t difficult, and setting an ilvl gap will not solve the situation because Blizzard is inflating ilvls so people wouldn’t be left behind.
Making mythic dungeons queueable also means that you need to put in an incentive to attract the better part of the playerbase to participate, which again is something that increases the chance of a successful run by a large margin, and it is why there are somewhat tangible rewards from LFR (runes, which better players in their prefered end game can use to give a bit of an extra boost to themselves.)
I know that you will say that this is just an opinion of mine, but a number of changes Blizzard has made in the past:
nerfing Cataclysm dungeons due to low success rate.
Dropping the idea of having to do silver proving grounds back in WoD as a requirement to enter mythic dungeons and bronze for heroic.
Making timewalking dungeons ridiculously easy and enforcing this with adjustments.
Just points towards the conclusion that Blizzard just cannot make anything that is both challenging and queueable. If you set damage requirements then people will start excluding specific classes, they will get frustrated with low performers, and said low performer’s mood will be ruined if he gets kicked for said low performance.
So if you take these into consideration then yes: Having mythic or even mythic+ queueable will directly affect the game for those who do not participate in it.
Also just a food of thought: How would you solve the system itself? The base gear rewards you get from callings/world quests is anywhere between 184 and 194, plus you have the covenant armor there which you can, with grind mind you, upgrade to 197 ilvl. The first mythic key level that rewards gear equal to that is a +6, so how will you give people incentive to do lower content than that? If you set an ilvl requirement to it then they will just queue for the first one closer to it and available to them. If you do set an ilvl requirement what will it be? Since a +6 gives 197 ilvl you cannot really set anything too high. 184 perhaps? that is already what normal mythic dungeon gear gives. Is it realistic then to expect people to jump from normal mythic straight into a +6?
Will you restrict with a progression system? Will people queue for it? Likely not, since they do not get gear rewards from it, so they will try to either look for loopholes to circumvent it, or just outright skip it and use the LFD instead.
Not to mention that at this point Blizzard cannot touch normal mythics, because the entire key system uses that as a base difficulty for its multipliers.
I am not for gatekeeping, but I just realistically cannot see a queueable version of mythic/mythic+ which can keep its current difficulty curve. It will have to be nerfed in order to accomodate to the fact that anyone can queue for them (since the only requirement you can realistically set is itemlevel.) Beside that all the effort they need to invest in creating an algorithm that can value in parameters other than ilvl and spec just sounds too much work for a company who already feels to be behind schedule with content creation. It is much easier for them to leave it in the LFD where people can make and adjust their groups to their needs and desires.
This is why people are so defensive about adding anything to a queue system. Mythic was created in order to give premades some difficulty (the fact that this is only relevant for the first few weeks is another matter entirely) while leaving normal and heroic queueable (which kind of shows on their difficulty levels.)