As a general rule, I'm strongly against adding seats to the Supreme Court. You raise several excellent, thoughtful points in your post. But in the end, to quote President Obama, "elections have consequences." Many Dems fully and completely blew it by staying home and not voting for HRC. This is the main reason Trump was elected and the Constitution vests full power in the President to appoint whomever he pleases (subject to Senate approval) regardless of how it leaves the political make-up of the Court. This is one of the multitude of consequences of the 2016 election. Blame Trump and his SCOTUS appointees on the Dems who stayed home on election day 2016. Hopefully history won't repeat itself on November 3.