I wish that someone would make a Wine-style compatibility library that would run a local websocket server that communicates with a lightweight Add-In using the new cross-platform JS APIs for Excel, and translates VSTO/COM calls into IPC messages that are processed and responded to. Then you could bring all the old VSTO tools into the modern world, running in a separate process from Excel. This is... rather daunting to even think about. Arguably Apple should sponsor the project as a way to bring the world of finance towards using Macs!