I was very involved with Denplan during its formative years and was part of the senior management team at the time, I would say that I have a good and thorough understanding of what it is and how it compares to other payment options.
Denplan Care is not an insurance policy. It does however have an insurance policy attached for emergency and out of hours treatment.
Your new dentist doesn’t understand capitation. Denplan Care is capitation, you are paying for your dental practices time, within which, certain treatments are included. It sounds like Your new dentist understands fee per item. There is a big difference. The former, when correctly understood and applied centres around prevention and understands that at times, treatment is the best option.
Generally horses for courses, depending upon your dental health and condition one will be better for you than the other.
My advice would be to seek a second opinion from a well established Denplan Care practice and then make your mind up.
IANAD.