Google Sheets is competing with Excel. Proton Sheets is competing with Google Sheets. So Proton Sheets is competing with Excel.

I used Word as a comparative example to say that parts of the office/docs suite are easy to compete with (there’s only so may things a word processor can do), while others (like Google Sheets or Excel, whichever order you prefer) is incredibly difficult to compete with; a formatting error on import of a Word doc is acceptable. An unsupported formula ruins the entire thing.