I don't get what's the point of others not attacking China in a war with India. Are you talking about them bringing all their jets into the fight?
That's not gonna happen, at least to the extent you are assuming. Simply because if 150-200 advanced jets defeat the IAF, then a whole lot of their scrub jets, drones and bombers will take to the air and pound Indian forces. If 150-200 advanced jets do not defeat the IAF, then they will simply retreat and focus only on air defence, where they will try and prevent IAF scrub forces from bombing them. There is no real need to add more jets from elsewhere in either case, since that's needed only in a long war, where attrition is very high. Basically, just a handful of expensive air superiority squadrons will determine in which direction the air war will go.
Also, a theater command is a simple numbers game. A terrain fits these many ground units. To defeat these many enemy ground units from the air, I need these many air units. And in order to ensure these many air units can attack with these many bombs with this much success rate, I need this level of air to air capability in these many numbers. And if I fail, I need these many units to prevent the enemy from doing this much damage to these many of my own ground units to hold this much of ground to consider it a victory.
This is pretty much how theater commands are set up. The commanders have already pre-decided what they need to win and how much they are willing to lose before calling it a day. But if the enemy is much more capable than planned, then adding more jets from other commands will simply mean more losses to your own side. So if your own command and resources from the reserve command are not enough, then you're gonna call it a day with a loss. It means you have severely underestimated the enemy. Although units can be reassigned to your own command, it's gonna happen in very specific cases, particularly when you are losing really badly and need more assistance, at least for the sake of buying time until you gain an advantage somewhere.
Theater commands are for rich countries. Which is why, while the Chinese have theater commands for all forces, we will have a single command for the air force, since our resources are not equal to requirements. But if the Chinese are forced to do this during a war, then it means things are going really badly for them.
