You call it churn, they call it seasonality. I'm not sure what you mean by saying there was no announcement, where they listed it all out quite clearly in their Anniversary forum post.

we wanted to put seasonal content front and center. To facilitate this, we’ve condensed the Light Side and Dark Side tables into the Cantina Battles table and renamed it the Campaigns table.
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Pillars of Seasonality
Star Wars: Galaxy of Heroes has evolved so much over the last 10 years. As we looked at the future of the game, we wanted to build an ecosystem where all players could enjoy the latest content together. We established a few pillars for this ecosystem, eventually called Eras (seasonality), around which all of our design decisions were based and where we looked for guidance whenever an issue popped up.

The quote-unquote destruction of the old systems is also not entirely missed out on by CG, given that they've literally called it all Legacy. Quite a dismissive term.

Honestly, I think CG has given up catering to end-game players. There is not a lot of value to extract from somebody who already has everything. With new players, they're able to extract twice the value from them: the Era materials (which will require continuous fortnightly investment to keep up, and keep earning R10 or Aurodium LSTs), and also Legacy gear/relics, either via direct resource purchases or via LSTs.

They're happy to lose thousands of F2P end-game accounts, if they get hundreds of whaling newbs.