"The engines, albeit 50+ years old, are solid. They have good engineering, a reasonable pedigree and just as important, there is an available supply."
The engines don't exactly have a great track record. It seems they blew up in testing every time the Russians used them and have never actually lifted a rocket into space. And that available supply is finite, once the old engines in storage are used up then they're done.
It really would have made more sense for OS to develop their own engines or partner with someone who builds new engines, not refurbishes old ones.