The key difference between VA and JBLU/SAVE/F9 in their initial growth phases was that the latter three all earned the right to grow from the onset - the models were immediately successful. JBLU was profitable as a startup in the early 2000's, while nearly the entire domestic industry was in the red. This allowed them to justify continued growth. SAVE and F9 have been wildly profitable since they were transformed into ULCC's, allowing them to justify further growth. You will note that JBLU, around 2013 or 2014, was struggling vs. the rest of the industry (though still profitable, I might add), and they too were forced to pare back the growth via orderbook deferrals - in theory, for every dollar that was invested in the company (via equity or debt), the company was generating returns that were lower than even the S&P500 index, meaning they were destroying value (at least, academically). Southwest went through a similar phase a few years ago (they wouldn't grow the fleet until they reached 15% ROIC), and if the SAVE saga from last year continues (generating lower returns on higher growth), they will get significant pressure on their growth plans.
VA, on the other hand, was not profitable until gas prices dropped in late 2014, and until that point was costing their owners a significant amount of money (before gas fell, why put money in an airline that generates no returns and looked like it wouldn't for the foreseeable future, when you could just park that money in the S&P500 index or even a US Treasury note?). The way to fix that is to slow the growth, generate profits on your existing assets, and then to start growing again, on more sound footing.
Scale is a a huge component of this business, but more often than not I think it's a misused buzz word -- carriers have to earn and continue earning to get scale - nobody will give a carrier a pass on earnings to reach a supposed critical mass 5-10 years down the line, when an infinite number of assumptions/scenarios could unfold and change the original outlook.
