These points are all so well-reasoned and difficult to argue against, I can't fathom how they managed to get all the way to this point without these things coming up and causing a rethink of the base approach. Quite incredible.
Isn't "launch from a plane" basically reusable first stage, 1.0? It was also more conservative, since the idea had already been successfully implemented in 1990: https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus.
Some quick googling shows Virgin Orbit started in 2011, and SpaceX landed their first rocket in 2015. Virgin Orbit's idea may have make a lot more sense before SpaceX proved out an alternative, and that didn't happen until maybe they'd done a lot of development that could have possibly paid off.
SpaceX landed a rocket in 2015, but Virgin Orbit wasn't even competitive with Falcon 1. Landing rockets has little to do with why it wasn't working out.
Yes without Falcon 9 the market would be quite a bit bigger, but even then its hard to see how VO ever gets very far. Its offering even compared to the other small launch competition isn't really competitive. Their offering was 12M for a tiny payload, their competition was often less then 10M for payloads with more flexibility in size and volume.
They simply have no migration path, no way to make their vehicle bigger, and pretty much every small launch company has realized that small launch isn't really a business you want to be in.
They managed to get a lot of financing pretty early on and managed to be one of the first to orbit, but their financial outlook was always pretty bad.