I'm planning a trip to Havana this January and as far as I can see all the online booking services are heavily overpriced - rooms which are supposed to cost $30CUC/USD are at least $60 on Airbnb. Keeping this in mind, would it be realistic to just find a good room on arrival to Cuba, without any booking in advance? Or would I be at risk of everything being overbooked by the time I arrive?
When exactly in "winter"? Because it's not the same november than christmas. If you are going off season for sure you'll find better prices when you arrive than in AirBnb or similar.
In my experience, even in August (when most europeans travel and also most cubans do), there are enough casas particulares. You won't have any problem to find one. Even you can find this kind of hosting for less money. I paid $15 for a double night all around the country. The trick was to enter into a "home network". I explain everything in this post.
So, the response is: off season there is no reason to book hosting in advance and on season you still have plenty of casas and you'll find cheaper prices onsite than online.
Keeping this in mind, would it be realistic to just find a good room on arrival to Cuba
I'd rather say yes. That was exactly my case when I arrived to Cuba in November. The only difference that it was the mid of November and you speak about winter, but I assume the otherness won't be critical.
I just picked a taxi, got to Habana Vieja and inspected couple of casas. The overbooked casas hosts called the friends, friends called their friends and etc. Finally the word of mouth made its job.
But the risk persists, so you should definitely have a spare cash, just in case.