I can’t actually find discussion I had with the Devs about it but I’m pretty sure 2 years ago the idea I backed most was a slight max cost increase. However, at the time there was a lot of talk about mythics not being strong enough to warrant the higher cost (something I unfortunately was a big part of starting: see here).
I remember big spenders back at the time found mythics very frustrating to run because at 15 cost (typically 17 with SS) they were a big investment and getting one-shot felt very bad when the players were making sacrifices elsewhere in their team. The Devs wanted people to feel like mythics were worth running and make them more of a straight upgrade over legendaries so they we worth spending so much more money on in the first place.
I think I basically agreed with this at the time but I was against making the second forms go down to 11 cost. I think I also said I was more keen for max team cost to be increased and things to balance out with time as stronger mythics were released, rather than the reduction to their cost. In addition to the cost reduction to mythics that’s also why they designed some lower cost monsters that were very good like the special rocks (which I already said above).