Microsoft Excel versions from 2013 onwards open every file in a single, merged instance unless you open a separate instance with
excel.exe /xor apply a registry key that requires certain updates.
Since I couldn't make the update to install, I just wrote a wrapper that creates separate instances. In order to use it, right-click holding shift and associate Excel files with ExcelNewInstance.exe. You can also pin it to the Start Menu and Taskbar in place of Excel.
P.S.: I could live with single instances if large sheets weren't prone to crash Excel and if the Undo/Redo stack weren't shared.