A lot of BTC will be lost over time but the rate at which it is lost will only decrease. People would have been careless with it in the early days because they wouldn't have considered its future worth. One big difference with BTC is unlike fiat - if someone else finds your lost BTC they can't just pick it up and use it, or if you die it can't easily be passed on.
I find it fascinating that over a long enough timeline, all bitcoins could be lost due to human error. As in, you have not provided a spouse to access your wallet. I'm guessing 200 years from now 80 - 90% would be lost if bitcoin survived that long.
It is possible, I just like to have a little more faith in the human race to preserve their wealth. For all we know there may already be much more than 4m bitcoin which is lost. If BTC is to exist through many centuries then I would predict high percentages of it would be lost forever.
if someone else finds your lost BTC they can't just pick it up and use it, or if you die it can't easily be passed on.
What do you mean they can not pick it up? Finding your lost bitcoin means finding the private key of the wallet and in that case they become the owner of the bitcoin.
I always keep my secret keys available to few people. Not the whole key phrase but the portions. My next of kin will be asked to find them and solve the puzzle.
What I mean by that is that unlike fiat it isn't just dropped to the ground and someone else can pick it up and use it. Or even when you are deceased it's not just a case of signing over the funds to someone else. They have to have the private key available to access the funds.