There's another workaround to make SharpDevelop support building projects using newer versions of MSBuild than 12, rather than editing your project files as Matt suggests, for anyone willing to download the SharpDevelop source and build their own version. It's described here. Although like Matt's workaround, this only allows you to build using MSBuild 14, it doesn't make the text editor understand new language features added since C# 5.
From what I've read in various threads on this forum, the main thing blocking the maintainability of SharpDevelop is its dependency on NRefactory, which is the component which parses and understands your source code so that the editor can prompt you with helpful suggestions, meaning that when new features are added to the languages, support for them needs to be added to NRefactory. It sounds like NRefactory could be replaced with Rosalyn, which is Microsoft's equivalent and which is now open source, but I read somewhere an estimate that this could take 3 months for a developer working full time who already has a good understanding of the SharpDevelop code.
I too would be very sad to see SharpDevelop fall into disuse as I've been using it since version 1.x and find it much faster than Visual Studio and less prone to random crashes too.
Simon