In your case, you can't blame the output transitors for the ESC's failure. Going to a 35 amp ESC isn't going to fix the overheating problem associated with a built in linear BEC. It might run cooler overall which helps but the problem is the type of BEC built into ESC's.This is copied from Dimension Engineering's website:It is very common for speed controls to have BECs rated at 2 or 3A. However, what the manufacturers do not tell you is that this rating is only true for an input voltage of 6V. The BEC on your speed control is what engineers call a 'linear voltage regulator'. It works by burning up excess voltage and turning it into heat. The higher the input voltage, the more heat gets produced. If there is too much heat, then the BEC will either fry, or shut down! The result of this is that in real world situations, if you are running a 3S lipo battery pack, your ESC's BEC will only be able to provide about 0.5A before it overheats. At 4S, most ESC manufacturers don't recommend you use the BEC at all, or at best power two small servos.