I was deep in the weeds of the ACA fight and this is not at all how it went down. Democrats had a filibuster-proof majority for only a few months altogether, and many in the caucus at the time were moderate red-state Democrats that didn't support a universal system. Hell, one of them, Joe Lieberman, had actively campaigned for John McCain and was on the shortlist to be his VP!

The ACA that passed via reconciliation was the most progressive possible version of it that could have passed; lose one senator or a couple of representatives in the final vote and it would have failed to pass (and arguably destroyed Obama's presidency in the process).