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.
You have a point there. Well as for me bitcoin will only be immortal if and only if the bitcoin that possess or acquire by someone can be passed on easily to other people after he or she died. It is indeed that people will die and will leave bitcoin as is but that doesn't mean bitcoin can be immortal.