The X-33 LOX tank wasn't composite...
it was made out of aluminum. My brother Steve helped build it. The VentureStar planned on using composite LOX tanks but it really never got that far.
The aerospike engine was fired on the test stand on even on the back of a NASA SR-71. That's a long way from being flight proven, but it was pretty close to being ready for flight test.
We spent about a billion dollars on the X-33. It kept getting heavier, slower, and later. If it was such a wonderful system, there's no reason why Lockheed-Martin couldn't have continued development using their own money. The fact that they didn't suggests something.