Watching peers of theirs who used training wheels, I've realized they're a trap. The mechanics of how a bike actually steers are completely different when you put training wheels on it. Whenever a third wheel is touching the ground (something that seems to be hard to avoid while turning, from what I've seen) it starts to steer like a trike instead of a bicycle. So transitioning from that to riding without training wheels is doubly difficult, because you also have to un-learn the instincts and muscle memory you developed with training wheels.
Transitioning from a balance bike to one with pedals is much easier because the main instinct they'll be taking from it - putting a foot on the ground when you get into trouble - remains useful. It naturally helps prevent skinned knees during the transition period.
One thing I think gets lost in the discussion of training wheels: people act like you have four wheels flat on the ground, with no opportunity to balance. But proper training wheels should have two or three wheels on the ground, depending on if the rider is balancing or not. In other words, the training wheels should be lifted slightly up: https://www.twowheelingtots.com/training-wheels-faq/