Of all the accessories to maintain backwards compatibility with, honestly the dock is probably the least important. Every single Switch comes with a dock, and very few people have bought a second. Nintendo don't even really consider it an accessory, as on their online stores (the only way you can buy a second dock), they sell them under
the "Replacement parts" category. People could conceivably use their old docks for [redacted], but they're probably going to have their old Switches in there, and in the majority of cases I'm guessing the old dock would stay with the old Switch, either sold, passed on to a sibling, sitting in another room, or just thrown in a closet.
It's a pretty big constraint to put on the design of a new device that may sell 100 million units or more for a relatively minor benefit. It would prevent the hardware designers from making [redacted] even a few millimetres thicker than the original Switch. It would even significantly constrain the internal cooling design of the new hardware, as air inlets could only be placed where the existing dock has made space for ventilation.
I wouldn't rule out the possibility that [redacted] will be compatible with existing docks, or even use the OLED dock directly, but if that happens, it's because there was already a decision made to use exactly the same form-factor as the existing Switch, and being able to use the existing docks is a side benefit of that, not vice-versa.
It would actually be very unusual for Nintendo to maintain exactly the same form-factor, though, from one generation to the next. Even in cases where they've kept a similar form-factor, like Game Boy Pocket to Game Boy Color, or DS to 3DS, they've made very clear design changes each time, either for ergonomic or functional differences, or just to create a clear visual identity for the new hardware. I'm guessing that Nintendo will likely want to create a clear visual identity for [redacted] as well, which may involve big enough changes that backwards compatibility with old docks isn't an option.
I'd wager that Joy-Cons will be backwards compatible (along with Pro Controllers, etc), both because lots of people have bought additional Joy-Cons, but also because Nintendo can keep supporting them via bluetooth even if you can't mount them on [redacted]. If Nintendo does make changes to how the new Joy-Cons mount to [redacted], it will still (presumably) come with a pair of Joy-Cons, and as you can't mount more than one pair on a Switch at a time, it's not a big deal if the old Joy-Cons can't mount onto the new hardware.