Unless you are getting absurdly lucky, the best you can hope for in the current economy is to keep pace with mythic releases, getting 1 to drop for each one that is released (legendary tasks push that up by a fraction of a point). I am of the opinion that mythic troops should just stop dropping completely after you have gotten just one. If not one, then definately four. If you currently have more than 4 of a given one, it can be rerolled into a different one for a nominal fee of “endgame useless” resources (souls, majors, gold) combined with gems. The alternative basically leaves collections incompletable, forever, without an extreme amount of luck, especially if you started playing after mythics were first released. And yeah, that may be the intention of the developers, but answer honestly: would you have started the game if you knew you could never get a full collection, no matter how much money you spent or how much you played?
As of now, this is mostly tolerable, as Mythics are mainly just fun little toys, and not getting one is not make or break to your ability to build a good and/or fast team and none of them make kingdom skill points possible (though they do make them slightly easier). If you can pretend they don’t exist, and you don’t “need” to collect them, then you can do an end run around the “complete” collection thing. However, if kingdom Mythics ever become the bottleneck to get a final star in a kingdom, especially a skill bonus, this is going to have a huge effect on gameplay and create an even bigger divide in the playerbase.
Also, yes, more than likely Mythics were created solely to be a drain on end-game players resources. However, there are other ways to drain resources other than through an endless RNG slog. An appropriate crafting cost (a few thousand gems + hundreds of thousands of souls, etc) would drain the same amount of resources, but take out the massive barrier for gaining mythics that aren’t in there exclusive period. On the other hand, it would make their cost readily apparent to all the uninformed low level players, which is, unfortunately, probably where a good portion of their income comes from (the fact that the first pack offered to new players still contains the Archmage armor pretty much confirms this).
With there being new ways to get (limited, and random) mythics through the shop in the upcoming update, though, a $50 a month player in a close-to-max guild now has at least the possibility of obtaining every single mythic eventually. So at least there is a path now, albiet with a paid component. Prior to this, in order to get another chance at a mythic you’d have to dump about $400 on average to get the amount of gems it takes to get a mythic on average (twice that before you hit VIP 5, which wouldn’t take long) and you could dump thousands of dollars into the game at one time still and not get everything (or even just the one thing you wanted). This is still the case with the new subscription pack, but since it is designed to make you wait and supplement what you gain normally, it is closer to a growth buff than just buying a cheat code. A step in the right direction, at least.