I agree, unfortunately, there is a large amount of Bitcoin that is lost, either in forgotten wallets, lost HDDs, passwords and the list goes on. It's inevitable, thus, the amount in circulation will always be less than 21 million. And to answer the OP's question, if you die, the possibilities are that nothing will happen to them. Nada.
Unless you leave the details, wallet file or whatever, to someone, I think it's practically impossible for someone to regain access. I haven't come across any wallet myself though, that lets you pass your BTC when you die, like others have posted.