It's usually just a swap. Swap Matsui out, bring Cornelia in. In 6 months, swap again.
That isn't true. Cornelia has only appeared twice, roughly coinciding with the 15th anniversary and 20th anniversary years. First appearance was from the September 2017 version update to the May 2018 update. Second was the May 2022 version update to the November 2022 update. That's it.
Matsui-P has also been available just twice (December 2020 to May 2021, and Nov 2022 to April 2023). Only the last one was immediately following a Cornelia exit, and we've never seen Cornelia reappear immediately following Matsui-P going away.
So yeah, Cornelia probably isn't coming back any time soon. Maybe 2027, please look forward to it!
All the cosmetics are stupid. Who cares.
If anything, I say bring on more cosmetics. In particular, why have they done cosmetic versions of lots of the Bonanza-only weapons but still haven't gotten around to others? If I can't get the actual Yagyu Darkblade, I'd still like a lv1 katana with the model for my lockstyle purposes! :)
I use the Silver Gun (cosmetic version of Exeter) a lot.