I presume you are referring to Leicester based on your profile, but it's worth noting that Leicester is far more competitive for NatEx because it's such a strong corridor that they want to keep hold of. If they don't compete and try to hold on to Leicester and Bristol corridors, NatEx might as well give up now.
I don't think Megabus was a strong competitor a lot of the time though. They tended to be the bargain basement option, generally had fewer journeys than NatEx, and served far fewer destinations. Megabus also tended to focus on cross country links rather than London stuff. I don't remember Megabus ever competing against NatEx with 11 buses per day between London and Nottingham, or 11 coaches per day between London and Leicester City Centre, or 22 coaches per day between Bristol and London. Yes Megabus was there, but I don't think they often were a major competitor.
Flix has come in, all guns blazing, and out to win over the whole network. That is a very different style of competition to what Megabus was.