To know how much bitcoin have permanently lost can be impossible for now, but probably after many centuries it will be possible, the untouched bitcoins can be assumed to have been lost, using addresses connected to it for the tracking. But right now, the amount of bitcoin lost can not be known.
I don't think we need to wait for centuries for something like that, probably the bitcoins that didn't move in the next 20-30 years top can be considered lost forever. I really don't see a single reason why someone shouldn't move a satoshi after 30 years, especially because he would be at least 50-60 year old at that point. Hodling till death is kinda extreme
I would guess that there could be some people, not a lot, but some people who purchased so that they could give it to their kids in the future. That could take about 20 years, even 30 years in some cases. If you do not have a kid right now, try to have one, then have one, and then they grow up, that could take as much as 30 years. This is why I believe that 30 years of waiting for something to be sure is not really a bad idea.
However, we are just in a bit over 10 years of bitcoins existence and we already know some of the coins are lost forever because of how it was sent to wrong places, plus satoshi had like a million which looks like it may not be in the market ever again.