Unfortunately the Republican legislature was in charge of changing the law -
and they refused to.
I think the court decision was reasonable, in response to the disruption of the mail/the delay in ballots arrriving/the county that got the wrong ballots. The Legislature should have passed a special exception to - but for political reasons they did not.
We always knew there was a possibility these votes woouldn't be counted - and all of the PR was to mail early and, if you couldn't, drop the ballot off personally. If I were advising the Biden team I'd say leave it alone - it doesn't change the outcome, and it is better if there is not a Supreme Court decision cementing the interpretation that legislature means strictly statutory law - not statutory law as interpreted by the PA courts.
In other words, appealing it could do more harm than good (from a long-term perspective).