Interesting. I do not think they will withdraw like they did in 1955. That did occur under extreme circumstances.a bit of both, Mercedes is suffering quite badly financially and what is happening in China will impact them quite heavily, there have also been a number of repuitable sources who hinted to the fact that Mercedes is looking for a way out of F1. If you add to that that I don't see how the other teams / PU manufacturers can accept another season dominated by Mercedes (who while doing a great job have also been responsible for writing the rules of the PU formula and had over one and a half year advantage over the other teams), enough is enough, Mercedes IMHO has 3 options after 2020 (I believe that they will close the current formula in style with the WDC and WCC): either retire in style or accept that it's now someone else's turn to take the silverware home or be responsible for the collapse of F1, I assume that they would take the first option, a bit like they did in 1955
I suspect they are good to at least 2023-2024. They have a significant investment which they will still be reaping the benefits of in 2021 and 2022, even under the budget cap. They have the right team in place with the right driver at its peak. I suspect they will stay for a few more years.
Now I would not be surprised if at some point they do decide to withdraw, switch to being only an engine manufacturer and sell off the team, but I think there would much stronger vibes on that if it was going to happen in 2021.