Re: "it is increasingly difficult to develop entirely new features"
> Calc is still far away from reaching parity when it comes to diagrams, built-in functions and formatting
I am not arguing here, but to me, with respect, that sounds like the surface cosmetic stuff, not the core numerical handling, which I have found perfectly competent.
Happily for me, the last time I needed to do a lot of significant spreadsheet work as part of my job was in 2013. Excel did the job fine.
But in the role I had in 2015, I did have to do a fair bit; I was working with a intranet web database, which had a lot of data but limited functionality. Often what I needed to do was run a query, copy and paste the result into a local spreadsheet and then work on that, where I could sort and search and so on, quickly and locally.
I had Office 365 and LibreOffice $CURRENT thanks to local admin rights.
If my selection was not perfectly rectangular, Excel 365 crashed on paste, 100% repeatably, every single damned time. Miss the last cell in the last row and there went Excel when I hit Ctrl+V. Including, of course, all open files in all windows.
LO Calc just took it, coped, and inserted it, and I got on with my day.
LO Calc saved me tens of minutes a day, every day, because whatever data I threw at it, in whatever format, it worked and handled it.
Excel is a prima donna which swoons and falls over if everthing is not perfect for it.
So, YMMV, because I haven't done any serious charting in years, but for me, in recent years, LO has worked better for me, including Calc, and it still has a grown-up UI for literate adults, as opposed to the cartoonish ribbon for those who can't read menus. I do use it and I actively prefer it.