There are a lot of opinions on this, and while I have significant experience in the board selection process, the building of the list of alternates is a combination of art and science.
The first board of 9 is complete, and there is a benchmark of average SAT or ACT, AFA scores, interview scores etc. The unknown is whether the people still sending in applications will be better, worse, or the same as the first board.
The fact that you have been told this early that you are an alternate tells me your score was VERY close to being a selectee. This doesn't mean you are a guarantee but it does mean the placement team at NSTC wanted you to know you are a competitive applicant. So does that mean if people didn't get an email that they aren't competitive? NOPE!! All it means is they were not as close as you were.
Last year, there was a pull of around 22% of alternates. This is not a hard and fast number, but it's something to keep in mind. What drives how many alternates get selected? The key drivers are how many selectees opt for a different ROTC program or opt for a service academy, or if there are folks who have clearly disqualifying medical issues. Another driver is the Tier I major. The limiting factor for Tier 3s is 15%.
So, I am not sure which is more frustrating.....hearing nothing back at all, or hearing you are an alternate but have to wait potentially until next March to find out if you get selected.
Hopefully this doesn't make you grind your teeth. We try to make the process as fair as we can, but there are just a lot of variables in the mix.