I think this is a good deal for both sides.. Looking at recent 5 WAR players like Bichette and Tucker, they made 4-5m in their first arb year, 12m in the 2nd year, and around 17m in the 3rd. That's around 35m total in arb. With inflation let's assume PCA could make up to about 45m in his arbitration years pre-FA.
That means the Cubs will effectively be paying about 70m (35m AAV) for Pete's first 2 free agent years, while the Cubs assume all the risk here. And Pete still gets to hit FA at a good age.
Looking at Tucker's FA deal, 5 WAR in FA at those ages is worth about 55m AAV, and will be higher with inflation when Pete is 29-30.