It does seem like a heavy engine. The low parts count could help make it more affordable and being an existing, in-production engine eliminates most of the R&D costs other than "man-rating". The J-2 and SSME are already man-rated but getting the J-2 back into production after all of this time could be challenging. If you have to develop an upgraded J-2 to meet the needs, then you're back to R&D. The SSME is an expensive engine to throw away and starting one in flight appears to be difficult for some reason. Those challenges could apply to the Vulcain 2 and RS-68 as well but I don't know if anyone has even considered it. They were designed for ground start.