I'm not aware of any table service restaurants in MK that hold back tables for walk-ins. Even the less than busy spots like Skippers Canteen can fill up. Last Sept, I watched them turn away at least 5 walk ins before I was seated for my reservation.
Your best bet is to 1) decide on a time and restaurant and make your ADR or 2) keep checking & refreshing your WDW dining app both the night before and also periodically early the day of your meal. You may find something that appeals. As I am less spontaneous when it comes to dining and prefer to know where I will eat, I make the ADR.
Another tip, if you can't find a restaurant inside busy MK, don't forget about all the nearby resort restaurants. For table service: Kona at the Poly, the Wave at Contemporary and Trails End Buffet at Fort Wilderness (take the boat). Also some very good quick service choices at these resorts that require no reservation.