I think you need to stop denting your rentals.
Never dented a rental, never been charged, but many people have.
How does excess insurance from a non-related third party stop your rental place saying "damaged car" and automatically charging you the excess, it's not really going to stop them charging you for whatever damage fees they have because the contract is with you and your credit card? Many of these places will only give you a car if you have enough funds for them to automatically charge a damage fee. Surely getting those funds reimbursed will take time with your third party insurer?
Sure but who cares? You get the money back. So it's a non-issue. If the rental firm wants to kick off about damage, it's fine, because you are covered.
Personally I get the damage waiver direct from the company
Then you are getting yourself properly ripped off as they charge an absolute fortune for it!
For example just done a quick quote for a 2 week intermediate rental with Europcar - to reduce the excess to zero, which is what the third party insurance does, is a staggering £367!!!
The same period of cover from the link I gave is £37.
So, by getting it direct from the company you are paying TEN TIMES MORE. It's a huge margin product for the rental firms and they absolutely love selling it because they make a fortune from it.
I suspect the reality is your view is theoretical and you don't rent cars, because I'm staggered anyone could advise just taking the coverage from the rental firm unless they didn't really understand the market.