I think in 20 years time any expats in Calis will be mainly those from today who are still there. Possibly a few who will make the move in the next 3-5 years but the numbers moving out is now on the decline and there is no reason to expect that trend to change. As you also mention, we are losing a remarkable number of characters from the "first wave" of retiring Brits in Calis and Fethiye in recent years. Also losing people in terms of returning to the UK due to ill health, and returning to the UK as funds run out.
That's the biggest challenge to the ex-pat community for the years to come really. The ability to live cheaply and pay for it with your interest is becoming less and less possible as prices go up and interest rates come down. I remember in 2004 a British couple telling me they had been living in Turkey for one year exactly that day, enjoyed a few Efes and a couple of meals out a week, and "had more in the bank today than we arrived with one year ago". It is this type of retirement that probably won't be possible in a few years time - certainly it will be possible for far fewer people.
I see Calis returning to being more of a captivating holiday destination and/or holiday home resort, and in the main the retired community moving down will be...Turkish.