It's not a replacement for HTTP per se. It's a transport that sits underneath HTTP. So HTTP 1.1 traffic will still flow, but over a SPDY link rather than vanilla TCP.
In this way, it can understand the HTTP traffic flowing over it and enhance it. For example, getting the multi connnections over a single TCP socket, which HTTP totally messed up with the aborted pipelining feature (all browsers switch that off, because it's broken).