To reply as to why there are evolution stones for Mythics (or any Dragon), that’s because not everyone that get them are higher tier or even spending a lot, so there is still the need of milestones to prevent a player in gold tier from having a Dragon overpowered for its tier, and motivate them to advance in the main dragon lines, not only Divine.
We are limited from having overpowered Dragons by many things, level, den, storage, or having to hatch a certain type of Dragon, and those limitations are there to keep a balance in the metagame.
As for getting a Mythic Dragon the way you propose, it would be nice for collection, but I really don’t see the point of having four useless trophy Dragons instead of one you carefully choose to fly with you for months and years til Obsidian tier and maybe beyond. I was told and came to the conclusion myself that one (or two) Dragon that you bring as far as you can, which means max stone or the further possible, was the way to go. I did it last year for Skarr, and got his Sapphire stone (max). I am now beginning to breed Platinum Dragon, so I still have two whole tiers to keep him. A year of a very helpful Dragon instead of a few months.
I am level 126, I retired my green Dragon shortly after hitting level 84, and believe me you get to this level sooner than you’d expect, especially when you know how much of a jump in power it gives you. Ask your friends, 84 is literally the golden level.
So my advice is this : even if the Mythic Dragon was available the way you want, it would still be a silly move to split your sigils and effort for several Dragons unless you are confident that you can at least max out the stones on one of them. That is, if you want to play the game for a very long time. As for myself, a year after I began playing I am still hooked. There are ups and downs in motivation, but I keep going back to it, because it’s fun and interesting!
… I hope I didn’t digress too much and kept my comment useful for the community and the discussion.