Presumably, the reason we have the Avoda in Mussaf is because of Uneshalma Parim Sefateinu - by saying (or learning) about sacrifices, it's as if we brought them. So if we're bringing real sacrifices we shouldn't need the second-best method.
Yet, we presumably had Mussaf, which is a mostly sacrifice prayer with little personal requests, in the time of the Beis Hamikdash and we didn't say that we are doing the real thing so why say a substitute.
So when Moshiach comes, will we say the Avoda?