I don't think it will be nearly as bad as many think. Yeah, games like Sim City, D3, WoW etc in the early days had problems, but you had hundreds of thousands or even millions of people playing.
If every last ED backer logged on at once, it's a whopping 27,146 people (at the moment). And of those 27,100-odd players, quite a few will probably be playing off-line. Even a year from now I doubt there will be a huge difference in the numbers. Compared to other games, Elite Dangerous is and will continue (IMO) to be a niche product. So unless Frontier has ridiculously small servers and huge multitudes of people buy the game in the next 12 months, I don't foresee a lot of problems from too many people trying to log on at the same time. Beta testing should have most of this sorted out as well by launch time..