My mechanic screwed up again. Is my transmission shot?

What is your question?

Recently I have my drive shaft boots replaced, the mechanic forgot to refill the trans oil. I realized that after driving the car for 2500 miles. Many times when it was warmer I would smell burnt plastic and strong heat coming from the car. I finally checked the transmission oil and it was about two inch below the cold mark when the oil is hot. The tran oil had dark and had little burnt smell. I just had the trans oil replaced and everything is running normal for now. My question, is my transmission shot? And how come the A/T oil temp light never turned on during the 2500 miles that I was drive on low oil?