I honestly don't plan on ever playing Cass or Skarner again after their reworks (I own Battlecast Skarner). The games ignore champ identity in exchange for something forced on that feels like Riot just wanted such and such in the game. Why is Riot so unwilling to rethink radical changes to champs?

Let's hold on the two specifics for a sec, if that's alright. I'd like to go into the mindset of why we update when we do.
First off, no change will make 100% of people happy. It's a sad reality, but true. People have preferences, and attempting to appeal to everyone will result in a poor product. That's not an excuse, mind you. We make efforts not to alienate players, and we make mistakes at times. We constantly have retrospective looks at updates we've shipped, identifying what we could have improved, what we did well, etc. If a change is actionable, we'll implement it past ship; but at some point we need to continue onto the next project, and after that and so on.
A large reason for larger changes to champions is when they live in a realm of unbalanceable. Certain characters are just unable to be balanced- they're either completely OP or completely UP, it has nothing to do with numbers. As I -roughly- understand it, Cassiopeia was difficult because her DoTs were incredibly hard to appreciate even when drastically overpowered. For Skarner, he lived in a weird space where you'd usually want to pick another tank over him, especially since most of his power lives in his ultimate.
I can't fully speak to Cass (not enough context), but for Skarner, we tried to add to his design. He still has his monstrous pick potential with his ultimate (to the fear of ADCs), but now has the added spice of the spires to augment his jungling experience. His gameplay has changed, but his identity should be similar?
I'm truly sorry to hear that you're not a fan of the new reworks. We'll continue to tune them now that they have balance levers, so hopefully we can find a happy medium between what you knew, and what they've become.
If you want more context on design, I'd suggest reasking the question towards @RiotGhostcrawler. He knows a lot more in that area than I do.

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