We can't say that with certainty, though... if the new console is thicker or taller in any way, then that would make joy con attatchment awakward, even if it was possible. I don't think nintendo will want to rigidly stick to the dimensions of the original switch... it just doesn't make sense to restrict themselves like that for the sake of being compatible with controllers that are notoriously terrible and people will do anything to get away from.
conversely, we can say that joy con will work wirelessly with the new system, because as long as it uses bluetooth, any controller will work. I wouldn't complain if they switched to 2.4ghz, but I doubt they would remove bluetooth completely so it's almost guaranteed, whereas physical connection is not quite as guaranteeable...
I am saying it with certainty.
As far as one is certain they can work wirelessly with the new device, I am certain they can and will attach.
The Nintendo Switch is not a GameBoy. It is not a PlayStation. It is a hybrid portable with detachable controllers. That concept inherently limits (and not for the worse) its dimensions.
If it already has to be small enough to be a handheld.
Big enough to dissipate heat.
And the controllers have to attach in some way for it to become portable.
If you have all those limitations on what it can and cannot be, must and mustn't be, why, oh WHY, would you restrict controller compatibility?
There is no reason. It is that simple.
Regardless of what anyone thinks, or believes, or wishes, Joy-Con have been a massive success for Nintendo. A gigantic one. One of their most successful lines of controllers ever. They didn't abandon GCN controllers. They didn't abandon Wii Remotes.
They will not abandon Joy-Con. As certain as the Wii launched with GCN controller ports, I am certain they will attach. You cannot sell people a console on the promise of a straight upgrade with controller compatibility and tell them to keep their old console around to charge their extra controllers.
They spent years of RnD on the Joy-Con and their rails, and even with OLED Model they were refining them.
Abandoning them would be at best an engineering failure, but more likely a marketing upset that nobody asked for.
They can improve Joy-Con, and I hope they do, without breaking compatibility, because what makes them work, what would make them compatible, the Joy-Con Rail, are already functional, already of an appropriate size, and already do everything they need them to do.