Was there much of a market niche for the 5150 once they started shipping XTs without hard discs? Even if you didn't want the hard drive, the XT offered more slots and a bigger PSU. It feels like they'd have to significantly discount the 5150 to make it a viable product by comparison.
OTOH, by late in the 5150's lifespan, they had six-in-one cards and HardCards so perhaps I'm overestimating the expandability concerns.