It is, after all bitcoins are eventually mined, because some private keys get irretrievably lost and those coins can therefore never be recovered.
- bitcoin tends towards 21 million mined bitcoin, but just like mathematical limits that means that it will continue mining some bitcoin essentially forever (progressively smaller and smaller amounts).
- people forgetting or losing their keys doesn't make bitcoin deflationary anymore than it makes the USD deflationary. Furthermore that is not something inherent to bitcoin, but simply a practical result.