Apollo was unsustainable anyway since it required so much workforce and thus money. STS is barely sustainable, leaving little money for substantial new developments. Just throwing more money to keep larger armies operating the current paradigm of space launch is not going to work. There will not be large scale space activities until space launch becomes routine. It is still in the unreliable, unsafe, expensive, and delay prone experimental phase. It doesn't need to reach airline levels but 98% reliability just sucks.
In the least, if you have decent RLV:s that can fly tens of times per year, every craft would on average live only a few years before being destroyed in an accident - it could never be sustainable.
NASA is still stuck in the Apollo mindset, just like Wernher von Braun warned. A narrow minded unlimited cash crash program where the government specifies everything. Afterwards everybody wonders what should be done with all the monuments that were built. The cash isn't unlimited anymore so you can only scrap them. Are you any better off compared to when you started?
What if you hadn't built such teraprojects in the beginning? Instead you could have developed and experimented with technology that would substantially lower the cost of space access (stuff like engine tech that doesn't have to be disassembled between uses. or multi use heat shield materials), then create a healthy sustainable small scale industry (perhaps suborbital) which could go through multiple generations of operationally improved craft. Small scale is essential to provide multiple paths and multiple generations with low cost. As you certainly don't always know in the drawing board what works out in real life, you build many vehicle generations and try many parallel things that are proved in operations, and each one is improved from the lessons learned, providing more value for less cost, often through things like better reliability and maintainability, not necessarily performance.
And then finally expand to real spacefaring.
It's a long road, but it's faster when you compare it to trying to continually leapfrog and abort, wasting billions every time and then starting from scratch again with new managers. X-30, X-33, you name it.
But rational discussion about space strategies is really hard, the Apollo legacy is just too strong emotionally. Large cathedral like rockets launched a few times per year capture the imagination but are ultimately the antithesis of sustainable spacefaring (which ultimately aims to colonization).