Getting something "for free" is hardly an issue since liquid oxygen is quite cheap as a percentage of launch costs (for example in case of Falcon 9, it's less t

Getting something "for free" is hardly an issue since liquid oxygen is quite cheap as a percentage of launch costs (for example in case of Falcon 9, it's less than $100k out of ~$28M of launch costs for a reused Falcon 9). Now you might argue that the resulting vehicle that needs to be carrying needs to be bigger, but then again, in practice we found it less complicated than hypersonic air-breathing propulsion, so whether the vehicle capable of usefully consuming it would actually be cheaper is pure speculation. So far nobody has been able to build even just one, much less mass-produce it.