ToD shall by the design has the different target audience.
Sure MA will stay. It may be even more people in MA when ToD arives (may be not at once, but after couple of months).
Success of ToD will depends on many factors. Despite it development takes so long, we still have high hopes for it, and trust HTC design. If it will be what I want and what I hope for, I will mainly fly it, keeping MA as a training arena. If it won't, I probably will cancel account, because I am burned out with MA and its gameplay model. MA is a great arcade arena, it has a lot of people, lot of good friends, great people, great pilots, often very fun fights, but just after 6 years in a game I simply can't motivate myself to fly seriously in it.