Could this be possible to survive that late into the game without that extra HP? No, that is why you don't see T70s in mid late games that often.
Have you seen any righ ranked games before? The t70 can survive into the late game and become quite useful with its sight ability.
Just because stuarts and luchs have higher survivability doesn't justify a health buff to the t70. The unit has better AI capabilities than both those units (until luchs hit vet 4/5) and it has a self repair which can be used while moving. In its current state the t70 is already quite cheap for its performance (manpower wise), imagine how OP it would be if it had 400hp with the self repair on top, it just doesn't work out.