First of all, we don't know everything yet, so I don't want to rush to judgment. But I've read and reread Fox's announcement on this. I'm mixed, but mostly pessimistic. Second caveat, I'd recommend everybody check out this old Extra Credits video on the topic of Power Creep. It's relevant. https://youtu.be/Bxszx60ZwGw Make note of any time they reference Magic the Gathering, as it's a functionally equivalent game (collectible strategy).
Social Point has long been dealing with the issue of power creep. At the same time, they're trying to balance the needs of F2P players while maximizing the profits they can get from this little game (a totally fair goal, people gotta get paid to make this game). So there are A LOT of moving parts in this.
What it seems like Social Point is trying to do is a soft reset on everything to solve their power creep, meta game issues, and some strategy decisions players are making. Obviously, once you've got several legendary monsters, any epic rarity monsters (or lower) become completely obsolete. By introducing mythic monsters, they will make legendaries obsolete (just like commons, uncommons, rares, and epics are). Fox has even said they intend to not make any new legendaries.
Everything is going mythic. This way, they can carefully re-craft what the most viable strategies are in the game. Fox said that they want to simplify combat. As time as gone on, battles have become more and more complex. As one monster becomes OP, a new monster is released to counter it. Then a monster to counter that one. Having a new tier of monster allows them to, essentially, reset everything.
- If you've been playing for years, none of your monsters will matter. It hurts.
- If you're a paying player, this REALLY hurts. All that money with nothing to show for it now.
- New monsters and NO PERMANENT STORAGE SOLUTION. Oh Lord, this is the bane of my existence, as a collector. Unless and until SP sorts this one out, nothing they do matters (to me!).
- Yet another currency. When I started playing, there was gold, food, and gems. That's it. Over time the following have been added: war coins, war medals, cells, elemental cells, elementium, elite souls, nemesis souls, and heroic orbs. Now we're adding mythic amber. I'm not a new player, but I imagine all these currencies would be very overwhelming for them. SP needs to eliminate some of these, change up how monsters can be obtained.
- A fresh start, everybody back at ground zero. SP has seen what this game and players can do, so they can be very intentional on how they handle new monsters.
- If you're a paying player, you'd probably keep paying for new monsters anyway, so are you hurt that much?
- Making legendaries obsolete could eliminate some of those extra currencies. They could make it quite easy to obtain nemesis or warmasters. They're legendaries, so who cares who has them?
This won't solve the power creep issues. It just restarts the process, delaying power creep. Yeah mythics will start simpler and whatnot, but then the next one will add a new feature. Some mythic monster will become OP. A new mythic will be released to combat that OP monster. It just starts it over again.
If they were doing this, I think they should have made a Monster Legends 2. Give existing players some kind of reward for what they've done in this first game, free monsters for monsters they already have, things like that. I know they'd risk losing some of their player base with that, so I understand why they didn't go that route, but I'm not optimistic about trying to have it both ways.
Also, GIVE ME A GOL DANGED PERMANENT STORAGE SOLUTION! I swear if they me something permanent, I'd roll happily along with just about any change they did.