I do not think it is due to myopia. I will like to give once a benefit of doubt to the Army, the MOD and the OFB here.
Here is my thinking., the nature of tank warfare has evolved. In fact the tank warfare evolved from WWI and was best exemplified as Blietzkrieg in WWII. After that the entire concept of regiments of heavy tanks fighting regiments of opposing heavy tanks have hit a stone wall based on changing war doctrines.
So going for AMX-13 light tank was not a bad idea, but the fear and panic created by Patton tank put paid to such an effort.
In 1965 itself PVC Abdul Hamid knocked out 8 tanks and the grenadiers armed with nothing but jeep mounted RCLs and mines destroyed overall 13-15 tanks. This is utter decimation of the concept of using MBTs. That is using the terrain to its advantage, a small and dedicated but very mobile team knocked out a company of tanks!
The overall battle of Asal Uttar reinforced the above several times. "Light" tanks like AMX-13 and "Medium" tanks like Sherman (which comprised the bulk of the force) decimated the entire assault. It was more like a turkey shoot. At the loss of 10 tanks, some 100 enemy tanks were destroyed
This was again reaffirmed in battle of Longewala. The post defenders with their RCLs claimed some dozen tanks. And bogged down the assault. The fighters finished off the rest in the morning. 34 tanks were destroyed.
All it shows is that heavy tanks show up as very slow lumbering behemoths against a nimble and fast moving enemy. In fact Khetarpal with his tank Famagusta and his dedicated fellow soldiers knocked off 10 tanks with his not-state-of-the-art centurions. Think through it, Khetarpal if he would have gone within 200 mtrs of the M1 A1 tank (that is right into them), he would have created havoc! The M1A1 stand off distance is 200 mtrs!! In his battle, Khetarpal engaged the bakis at extreme close range (<100 mtrs).
In fact, in 1971 one can say that Indian Army tanks swam to liberate Bangladesh. Compared to behemoths like Patton, the PT-76 used would be ultra-flyweight categories (coming in only at @15 tonnes!). The Indian blietzkrieg was helicopters and floating tanks. https://medium.com/war-is-boring/indias-armored-cavalry-rolled-and-swam-into-bangladesh-90b3281b2099
Not that the 50+ tonne battle tank has completely outlived., its need is still going to be there. Though it will not be the "Main" Battle tank. With mobile warfare, Rudras with Helinas and LCAs with guns and rockets can decimate any battle tank company in the open easily.
The very role of the tank is rightfully questioned. The tank warfare evolved over the plains of Europe and N. Africa and may not be replicable every where, particularly when faced with a very nimble adversary.
The T-14 armata is an entirely evolved concept. It is an automated cannon with minimal crew and extreme protection of crew. The cannon is a huge 125 mm and completely remotely controlled machine guns. Its main 'weapon' is "Afghanit" which protects it from incoming projectiles actively.
In effect a very modern 'medium' or even 'light' tank with <50 tonnes can be both mobile and effective and at the same time can be air transported as needed.
I think the next step for Arjun Mk III is to actually shed some weight., a redesign is in order with more automation. And there is nothing wrong to it, western nations have not introduced any new major tank design (the last one being the Leopard 2s/M1As).
IA already has T-90s and Arjun Mk2s. It can form a core for the next decade or two while IA transitions to more lighter tanks with less tank crew and more active protection.