Could you explain this opinion further? I was under the impression that there were a number of benefits to this type of launch (the US has used it for experimental designs before, and I believe it was one consideration for the shuttle). Also, why is the entire program dependent on that one aspect of the design?
So with assisted launch, there are a number of real advantages, and there are some advantages that turn out to be overhyped or undercut by some disadvantages that the tech has. TLDR, it has a niche which includes gimmick "space" tourism and maybe small or specialized satellites, but doesn't look likely to be very useful as a mainstay of spaceflight for the coming decades.
I'll start with the good. Assisted launch involves putting your rocket on a plane and then flying it up as high as you can before you actually launch the rocket. This means that instead of needing specialized rocket launch facilities (of which there are maybe a couple dozen in the world), you can use any airport of sufficient size. You can also fly your rocket to the optimum location before launch. Third and maybe a little surprisingly, the fact that you fly above most weather before launching means that you are not at the mercy of weather conditions, which accounts for delays and risk for probably most traditional launches. Assisted launches are also better at launching at short notice.
All this combines to give assisted launch great flexibility and the capacity to launch more times of the year, into more orbits, from pretty much anywhere you want to go. For very specific small packages which need unusual orbits, this can be a boon. For national security packages that need to be able to launch quickly, for example if your opponent has anti-satellite weapons and you want to be able to redeploy on short notice into any necessary orbit, this is especially huge.
Now with the overhyped. Some people bring up that launch assist allows you to launch your rocket at a higher altitude and with some velocity already imparted by the plane it is based from. This essentially uses the plane as a pseudo-first stage for the rocket. This is all true, but it turns out that the boost given by the plane is miniscule compared to the fuel needed to actually get a payload into orbit. A plane flying at 14 km above the planet at 300 meters per second just isn't that big a deal when you need to get up to 200+ km and 30,000 meters per second in order to keep the payload from falling right back down. And you do pay for the ability to launch from a plane by needing to make the rocket much smaller. Generally, in rocketry, the bigger your rocket the more efficient it is, for a bunch of reasons including the square cube law and aerodynamics which we won't get into here. Suffice it to say that using a plane as your sort-of first stage ends up being a bad deal compared to just using an actual rocket as your first stage if your goal is to get mass into orbit as cheaply as possible.
On top of all this for Virgin Galactic specifically, the vehicle never actually reaches orbit. You go up, you freefall for a few minutes and experience apparent null gravity, and then you land. You can never for instance dock with a space station, or transfer to another craft, or spend any longer than a few minutes in "technically space according to the airforce". So while it's an interesting novelty, it will never be able to hook in to what many of us anticipate will be a future space infrastructure, including civilian space stations or orbital industrial facilities or transfer flights to the moon and beyond. So in that way it looks likely to be a dead end, barring some serious improvement in the technology.
Hope that makes sense, and sorry for the long post!
