The general idea is that 1st form and 2nd form movesets are weaker enough than the final form that people should not seriously consider leaving them unevolved. There are, of course, a few exceptions like Mojinator where the moveset changes dramatically.
I’ve been a big advocate of 2nd form nerfs on the problematic monsters, pushing the Devs in DMs. Most of them have been dealt with over the years. This is the best solution for dealing with the problem. The Devs intend to make people want to evolve their monsters.
Some extra info about mythics:
I can’t remember for certain, but I don’t think mythics get a defence buff in their second form. However, there might be something with the difference in stars between the monsters attacking. E.g. Legendaries attacking a 2nd form mythic do normal damage but if an awakened mythic attacks a 2nd form mythic it may do less damage than it would when attacking any final form monster. When I did all the above calculations, I didn’t have mythics to test this with so I cannot say for certain (what I’m writing here is just a random theory).
I say that because I think with mythics it makes proper sense to awaken them to make them survive better, even ignoring how you can give them HP boost only when awakened.
One thing that definitely did happen was 1st form mythics gained a lower form defence buff. Since this defence buff is meant to be for monsters without fully trained stats, it was ridiculous on 1st form mythics that were trained to 100%. Here’s a post I made about it, looks like it was a 0.4x damage multiplier to what they took, so with their roughly 57% of the awakened form HP they took 1.43x the amount of damage to kill. Once a few mythics had come out that showcased the problem in a big way (Plumesilisk & Lemon), they made almost all 1st form mythics get terrible movesets and tweaked the code so 1st form mythics specifically get the damage they receive multiplied by a large amount (e.g. 2-3x). They did that as a hotfix of sorts, but didn’t change the overall calculation.