Bear in mind SCOTUS ruled against South Carolina Democrats yesterday and overturned a federal court ruling that waived the necessity of a witness's signature on that state's mail-in ballots, but ruled the ballots already sent in without a witness should be counted. They didn't list any dissenting justices and three justices voted to throw them altogether (Thomas, Alito, and Gorsuch).
Bottom line is that decisions regarding ballots and elections belong to state lawmakers and if those lawmakers choose to ignore medical evidence and scientific advice, so be it - federal judges can't second-guess any decisions that state lawmaker's make to address COVID-19 and the election (or opt not).
The distinction in Texas is that it's the Governor's extension. It will be interesting. To your point though, a challenge to the extension might meet the same fate insofar as those votes cast before the 19th, so why take a chance.