If you're using end-to-end encryption, it doesn't matter that your traffic is being monitored (I mean, that's an oversimplification, as the presence of large amounts of encrypted traffic is notable in itself, but that's outside the scope of this comment).
A VPN is useful in settings where you're dealing with a malicious ISP (for instance, ones that hijack unencrypted HTTP sessions to inject their own HTML) or any untrustworthy third party network. Do I trust my VPN provider more than my ISP? Yes. Do I trust my VPN provider unconditionally? No. That's what end-to-end encryption is for.
A VPN is useful in settings where you're dealing with a malicious ISP (for instance, ones that hijack unencrypted HTTP sessions to inject their own HTML) or any untrustworthy third party network. Do I trust my VPN provider more than my ISP? Yes. Do I trust my VPN provider unconditionally? No. That's what end-to-end encryption is for.