What ever next major war is going to be, i'm sure that heavy tanks (+50 tons) will become like battleships during wolrd war 2, well armored and impressive on peace time activities, but in real shooting war those things are gonna be largery used as target practice by more flexible forces coming from all directions.
Had they put 125mm gun on Type-15 with room for 130mm gun update it would have been able to replace all other tanks in PLA service.
Nah. Type 15 is flexible and all but heavy armour can take a beating something like the Type 15 just can't. That's worth something. I doubt a 125mm L48 will go easy on the gimbal and hydraulics, which means slower and less steady gun. Who knows though. Do they have room? I'm sure PLA would have preferred a L48 125mm similar to one on the Type 96 or Type 99. It would have one if it could accommodate, but it doesn't.
The purpose of the Type 15 is to fill a certain niche where any battlefield it can go on while heavier tanks can't, mean the Type 15 is the heaviest armour and biggest gun around. I think one of the more vocal Chinese military "analysts" summed it up fairly well as; anything you can deploy on the battlefield (high altitude or requiring light weight) cannot defeat me (Type 15), anything that can defeat me, you cannot deploy onto this battlefield.