nearly all the really good disco was commercially successful and in the charts
well, no
depends what you mean by "in the charts". that's taking the absolute strictest view of true obscurities (some of which have since been made famous by modern DJs).
if extended to anything not certified/top 20 hit, there are dozens and dozens of tunes every bit the equal of Chic. that's not a slight on Chic. I'm a massive fan of Chic - as well as Salsoul, the Philly sound, Bohannon, whatever famous high in the charts disco one cares to name. especially for disco I'd agree charts have some worth as discerner of quality, if certainly not to the infallibility of free markets level you're suggesting. there are so many massive club tracks that weren't really chart hits - especially as you get into the 80s with the Garage etc. I trust Levan, Mancuso, Gibbons, et al more than the charts, acknowledging there's plenty of overlap. at that point it also starts depending more what is included in "disco", and I'm a very strong advocate for an expansive definition.
but still: the idea that there's nothing as good as "We Are Family" is simply wrong, easily demonstrably so
citing one decent early hi-NRG tune - that's Denis LePage of Lime, MTL being a huge hi-NRG hotbed - isn't a strong argument