Niki Lauda is on record saying the problem is not with Bottas, but with the car. It's designed around Lewis and Bottas prefers a different setup. Reminds me of the Schumacher / Barrichello years when the Ferrari's were designed around Michael's driving style and Rubens had to try and adjust. As such, they may keep Bottas as he will have had multiple years to get used to the design.
As part of them becoming the title sponsor, Aston Martin will replace Tag Heuer on the engine badging for the Renault engine. Speculation is Red Bull will also adopt Honda engines in 2019, though not sure how Honda will feel about them being badged as Aston Martins, though "Aston Martin Red Bull Racing Honda" is kind of a mouthful.
As for developing a new power unit for 2021, that could be something Red Bull and Aston Martin do using Building 9, though the rumors persist the VW Group could provide one, as well.
Sounds logical as they're seeing multiple turbo failures and that would be one of the most-stressed parts of the engine. On the plus side, I can now just run the races in the background since Lewis is guaranteed the WDC now and Mercedes has the WCC locked-up. Only real battle now might be whether RBR passes Ferrari to claim #2 in the WCC.