Uh, schools are currently flipping to all remote learning overnight when they have positive tests in their schools right now.Snow days might still be a thing. Teachers can't just flip a switch and move everything online, if the lessons was prepared with the idea that it would be taught in class then it can't be expected for them to switch everything over in, what, an hour or less? Depending on how far ahead of time the school knows the weather will prevent kids from making it to class. Then there's the issue of some classes being near impossible to replicate online (like any science class with a lab) and some students not having access to WiFi at home. Online learning has its advantages but I doubt many teachers and schools will be eager to make much use of it.
So yes, they can just move it online instantly, its happening daily here in CT.
And in the places with hybrid models (like most districts here in the Northeast) all lessons are currently being taught to both in person and remote students. The issues you're bringing up are issues now, regardless of whether its a snow day or not.