As long as rho does not change we can predict with a precision to a Lovelace how much ADA will be in reserves.
Yes I made the chart myself. I track everything in a nice excel sheet so pulling this data isn’t that hard. The supply cap is there in the genesis and is equal to 45B.
Treasury is a separate “account” that collects 20% of emissions+transaction fees+unstaked rewards+return differences between pools introduced by a0. This account has no owner and its funds can be accessed through voting.
So rho reduces reserves at diminishing rate. This means they never get depleted actually unless there is one final payment at the end. Realistically you could hit a time period when the reserves are so small compared to tx. fees that they would make no difference and that could be when you would just stop their emission.