How does XDN cap at 8 589 869 056 in case where large proportion of network stakes for long periods of time?
Time of the entire XDN distribution and reaching max xdn value is much closer, than 77 years, but any way we make a system architecture to be decentralized and "self-maintenance". In general the idea is to keep 150 block reward for miners
forever, but still have max XDN number.
You may ask how

"Burning money"

Oh, I am. I'm only burning my half. [throws the Chechen's cigar onto the money, setting it ablaze] All you care about is money.
Joker
Seriously, lets burn like half of a fee to reenter XDN in to the main supply. This can be started when need, miners will anyway have their Proof-of-work reward, but the idea is to go to the Proof-of-activity design to make network the most secure Proof-of-work alike solution and further more energy efficient in terms of network protection.