The 2 prototypes weren't prototypes in today's meaning of the word, they were essentially experimental supersonic passenger aircraft, in mid 1960s when their design was frozen a lot of work needed to be done before you had a mature SST.
The next 2 per-production were nearer today what we would call prototypes for a design, much much closer to the final spec of systems and design.
The 2 first production aircraft, after re work and re certification could have been sold on to operators should the market have been there.
The pre production set could have been dropped in many peoples view, but due to the way the design was maturing the EIS would have been delayed a few years, and there was simply a requirement to allow the next stage of testing to move on.
Very much like the NASA moon programme, they could have taken a delay and flown more apollo missions and completely dropped Gemni, but at that point in time they needed to be flying to get the data.