Smart move; any OSP design either needs a STS-like ET/SRB system or would have to be towed up to high altitude before launched [see X-15]. The former system is, generally, what NASA seeks to move away from; the latter hasn’t been done for decades.
I think this is a sign of a return to the crew capsule days. While NASA-philes [and the greater American public] might consider that to be outmoded, it’s certainly been effective for the Russians for the entirety of their spacefaring days [which number more than 40 years, for those keeping track].
NASA also noted that an STS orbiter modification would not be carried out as originally planned. In both Challenger and Columbia, the crew survived the initial infarction and perished as a result of the impact of the crew compartment. Design modifications were being considered to make the crew compartment sturdier, but with STS on the way out, it doesn’t make fiscal sense when the risk of crew loss is so low [<2%] and the development time is long [4-5 years, at best, with every modification requiring a re-fit that would probably take a year], almost as long as the planned life of STS.