But I don't think it's a very reliable way of determining the amount that is irretrievably lost, many have no reason to move their coins. I can take myself as an example, let's say I bought 1BTC back in 2015, and did not move it from then - is that lost coin?
Not reliable at all. It reminds me of a situation where I'm not exactly sure if it was this or last year, but there was an address with thousands of coins in it that people assumed were lost due to years of inactivity, but it turned out to be one of Coinbase's older cold wallet addresses.
All these 'x million lost Bitcoins' clickbait headlines are the result of news outlets trying to spin up a narrative to convince people of how scarce Bitcoin is. I don't even consider satoshi's coins to be lost. It may very well be that he is THE holder of last resort. Waiting for the day Bitcoin becomes a reserve currency and he directly the wealthiest entity in the world.
It is that the bcash fork made me move my cold wallet holdings in 2017, but if there was no fork, I today would have had a few cold wallets with 5 years of inactivity, and they would remain inactive until the price of Bitcoin hits $100,000.
Also don't forget the funded 2011/2012 casascius coins that news outlets are blatantly blending into their total number of "lost" Bitcoin. Currently there are 3 active 1000BTC coins, which according to "research" of news outlets are lost too.