I'm not an expert in cosmology, but from what I know, the "big rip" is based on the assumption that the cosmological constant in Einstein's equations is time dependant, and that's not a justified assumption. I didn't watch the video, but from other videos I've seen of Lawrence Krauss, he usually says that the final fate of the universe is entropic heat death, so I don't know if you took that to mean the big rip.
Heat death is actually not really the death of the universe, but instead it's an inevitable state of the universe. There is a concept called "entropy", which basically is the number of ways that a thermodynamics system can be arranged. According to the second law of thermodynamics, entropy tends not to decrease (ie, the early universe had less ways in which it could be arranged than it has today). The consequence of this is that the universe, when treated as a big thermodynamic system, tends towards maximum entropy. But the second law of thermodynamics is a statistical law, so entropy can decrease at certain times. So interestingly, given an infinite amount of time, due to those thermal fluctuations, every possible state of the matter in the universe will be realized an infinite number of times.
I've thought about it and I can't even comprehend the universe having an end, it just doesn't make sense to me. But the thing to remember is that experimental observations are the final judge, so just because it doesn't make sense to us doesn't mean it's wrong. There are plenty of things in physics that are experimentally confirmed which don't make sense to us, since we've only evolved to make sense of things in a limited range of parameters (moderate sizes, low speeds, moderate gravity, etc.).
