I have both CSR (net fee $150 because I use the $300 travel reimbursement) and Personal Amex Plat (net fee $150, because I use the $200 Uber credits and the $200 airline credit - ignoring the Saks thing for now). CSR for spend benefits, Amex Plat mostly for non-spend benefits.
CSR's checked bag delay/loss insurance and trip interruption/cancellation insurance is of great value to me because it covers redemptions (so long as the taxes/fees are charged to CSR). If I had to buy my own travel insurance for each personal trip, it would cost more than $150 per year. That, plus the car rental LDW being primary, is a strong argument to keep (ie not cancel) the CSR.
If I have the CSR already, what additional benefits do I get from Amex Plat?
I put very little spend on my Amex Plat (less than $20K annually). I use the Gold hotel status (to help get occasional late check-outs), Centurion lounge access, and concierge service. So, to me if I already have the CSR, it's a discretionary card with additional luxury benefits (over the CSR) that come from membership rather than spend.
To someone with a CSR considering the Plat as an additional card, it comes down to whether you like spending $150 annually for luxury. If your home airport has a Centurion lounge in the terminal you typically use - for example many CSR cardholders in the Bay Area fly United out of SFO T3 - the answer may well be yes.