Ring Protection Plan Plus + Existing Doorbells and Ring Devices

I'm a Big Time Fan of Ring. I already have two Ringdoor bells and a Floodcam with an account that is charged $100/year for the service of video storage and review. I already have an ADT Alarm system that has worked flawlessly over these past 15 years. I'm not going to disclose what I pay a month for their service, but let me just say its A LOT! Now I see Ring has their own Alarm System that I am seriously looking into. What I'm questioning is the monthly cost of this Ring Protection Plan Plus, including the three Ring cameras I have already. Seems to me that if I ditch my ADT alarm system, and replace with this Ring Alarm System with their Protection Plan Plus, I'm going to be saving some serious $$, and that's including my purchase of a new Ring Alarm System. What Am I missing here? Also if/when I get this new Ring System with the associated Ring Protection Plan, will I also be saving money by bringing into the Plan my existing three Ring devices? Thanks in advance.....