Mono is not a direct superset of Core, but yes if you have an existing large .NET application you might have an easier time working with Mono than Core. Some of that may change as Core starts to get closer to .NETSTandard 2.0, depending on what sort of a legacy codebase you refer to. (That is, Mono has an okay-not-great emulation library for System.Windows.Forms, but it seems like it may be unlikely for .NET Core to have anything similar.)