It is also possible to land on the Moon, we know that. But no one else tried for more than 50 years.
The barriers to becoming an agile space company are formidable. When looking at SpaceX, we forget how many space startups from that period are now defunct and forgotten. John Carmack founded one, too; it is gone.
Carmack's was very much a part-time venture. It was interesting to read their blog and watch their videos. It was clear that they were very good at certain things (code + welding) and not very good at others (electronics, fire-proofing, planning, testing, finite element analysis, any kind of simulation that Carmack didn't write).
An interesting fact is that they did very well as long as they could conduct cheap tests very often. As their rockets got bigger, they could no longer do that and then they floundered.
The barriers to becoming an agile space company are formidable. When looking at SpaceX, we forget how many space startups from that period are now defunct and forgotten. John Carmack founded one, too; it is gone.