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You can use inactive UTXOs, and different age cut-offs to estimate possible lost bitcoins. With different age cut-offs, you will have different estimations and they are all not accurate.
https://www.bitcoinmagazinepro.com/charts/hodl-waves/There are always very old, and inactive UTXOs suddenly become active again and make your estimations inaccurate. It's only important by knowing a fact that there are lost bitcoins that contribute to higher value of Bitcoin and it's gift for all of us.
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.
You have essentially stated what I already mentioned. You call it inactive UTXOs, which essentially add to total BTC coin age. But a dormant wallet is not a lost wallet as this here shows:

The post is from April 2024. Now it is more or less safe to say that most of these 50 BTC wallets are lost because those were probably some nerds playing around mining a block and then lost interest and did lose or forget about the keys. But there is no way to say how many of these wallets are indeed dead in the water.
When I read this
I think the estimation about lost wallets is way to aggressive. Asserting that wallets with unspent UTXOs since 2014 can reasonably be considered lost is a joke in my opinion. 2014 was the time when people started figuring out all the stuff about handling wallets and so on because the info was out there, Youtube channels popped up, guides were there and so on and so forth. I understand that those who experimented with mining in 2010 may have lost the coinbase transactions here and there because they lost interest, hence the many 50 BTC wallets, but not in 2014. Coins have still been lost since then of course, but I can tell that those who have been around had a very good clue how to safely store BTC and those who newly joined had access to essential info.
What's more, BTC had crossed the $1,000 mark already in 2013. Everyone knew that this is more serious than buying pizzas with 10,000 BTC. There is no way it can make sense that Chainalysis defines wallets as lost when coins haven't moved since 2014. I don't know what you think, but to me it is ridiculous and I am honestly not sure why they would come up with that idea. I'd just call them dormant and let everyone else come up with their own opinion. I don't think Satoshi's coins are lost. It's possible, but I assume they are not because why would I bother? It makes more sense for everyone to take them into account rather than optimistically write them off the BTC supply balance sheet hoping that it boosts the value of my holdings due to increased scarcity.
Edit: Here is another example.
Wallets from February 2009 waking up after 15.5 years.
I agree with a lot of your reasoning about points that you are making @tiCeR especially when it comes to the difficulties of making inferences from the information that we know versus the information that we don't know, so no matter what, there is a lack of solid evidence if we are going to conclusively draw inferences on the non-movement of coins - yet at the same time, I largely come to the opposite conclusion as you in regards to my own thinking that the number of lost coins are likely higher than the estimates, and you seem to be coming to the conclusion that fewer coins are lost than the various inferences.
First, personally, I believe that Satoshi's coins are likely to be lost "on purpose."
Secondly, I believe that a lot of bitcoiners likely believe that they have coins, but they are not in a frequent of enough practice of accessing their coins to realize that they really don't have the coins that they believe that they have.
Third, I believe that a lot of folks are not very good at their plans of successorship, so many of them have not adequately communicated the successorship of their coins, and even if they have communicated such successorship, the form of their communication is not clear enough (or updated enough) in order for successors (namely heirs) to be adequately able to know about the quantity of their coins (and the various locations of such coins) and to access such coins. Password management can be quite difficult, even when we are trying to recall our own accounts and/or locations of coins and other important things, so frequently if heirs are not even able to adequately figure out coins that a deceased person might have had, they might also not be ready, willing and/or able to spend time trying to figure out such puzzles...even something as simple as getting into the deceased person's laptop that may end up giving some clues, there can be difficulties getting in, and then also difficulties sorting through what information might be important, while perhaps attempting to preserve some of the privacy of the deceased person.
There may be deceased person personal papers in electronic form, in paper forum and even stashed away in various places (buried stamped steel plates that might not even have the whole seed in one place.. so having to put 2 or 3 parts together)
Fourth, - somewhat not related to lost coins, also consider the logistical problems of having someone technical (who might not be 100% honest) helping out a non-technical person, and if the person comes across information that reflects value or even bitcoin that can be irreversibly moved, and if such technical person comes accross life changing amounts of money that he considers himself to be more able to meaningfully spend as compared to a potentially undeserving and unknowing person who is the rightful beneficiary, then surely there can be temptations to move some or all of such coins to their own self-custody, yet theft of coins still would not be the same as lost coins, since in this kind of a case, the coins would just end up going to someone who took the coins... and sure maybe he does not take all the coins, so he ends up splitting the pot or maybe even retaining some control over the coins when he might transfer the coins to the intended beneficiary.