It seems to me that in the interwar period there were a lot of tanks designed with the idea that they would stay with groups of infantry, providing direct fire support while being a lot more durable than a field gun. My understanding is that this was generally abandoned in favour of faster tanks which operated somewhat independently of infantry. But to my very limited knowledge, the infantry tank seems to make sense. What were the theory’s disadvantages? (Or is my understanding flawed?)
I think your premise is a little off. Tanks generally are trained to operate alongside infantry in what is called combined arms tactics. This allows the tank to do its job while the infantry can protect it, and vice versa. This is especially useful in urban environments. Combine infantry with tanks, IFVs and support vehicles and you have a flexible and formidable force.
On top of that, rather than making t different anks for specific jobs, we tend to prefer making them multirole. This is the current trend with most land and air platforms. Notice there really aren’t a lot of designated attack planes anymore. We tend to prefer multirole airframes that can act in an air to air and CAS/ground attack role. It adds flexibility and simplifies the supply lines and training. The Abrams, and other nations’ MBTs are perfectly capable as main battle tanks as well as infantry tanks. No need for a second type of tanks with different parts and needs.
When they develop the tank jet it’s game over
It’s called the A-10