I was on the fence about Plagueis, but:
1. There was at least one line at the end that stressed straight-forward: "The Supreme Leader is WISE (Darth Plagueis the Wise was his name)
2. The only reused score beyond the obvious on the official soundtrack is the end of Snoke's theme. Why in the hell would they direct copy the same Plagueis theme from Episode 3, if it isn't him?
3. It would connect all of the movies together perfectly. Otherwise, there is no reason that a random dark side user, seemingly more powerful than Palpatine, arose within 30 years of his death. In the canon, Palpatine sent troopers to far reaches of the outer rim in search of mysterious dark force. It is pretty obvious this is his old master who has been described and shown as scarred and damaged.