IBM mainframes do the same thing with COBOL because modernizing core code means having to notify and test all the downstream users that have their own code to deal with the legacy issues in the current stack. I was once explained by a retired COBOL programmer (who spends his summers contracting for maintenance) how this tangled mess of thousands of banks feeds in all the way to the US central bank to determine interest rates - in some extreme cases, IBM still has to emulate hardware bugs as to not change expected behaviour. This is not something you just migrate away from or fuck around with. There's a reason IBM still sells mainframes that runs code from the 1960s.
But for other uses of Excel, I can say it can be easily exchanged. This is why I used 99.99% percent, but not more nines. The remaining "three nines" are the niches you're talking about.
It's like how we say "Nah, it's a small set of matrices. just 2, 3000x3000, double precision floating point matrices, and a couple of vectors and some multiplication (i.e. in a range of a couple of billions)".
Libreoffice Calc also supports the same thing over ODBC AFAIK.
It's true I'm not a spreadsheet power user - but not many people are.
I forget about it myself as I've worked for modern tech companies since 2008, but outside of that world is a very different place where old ways of doing things stick around. Anecdotally, about 2 years ago I got a cold email from a co-op job that I had in 2002 for a manufacturing company that I wrote a perl script for asking for help modernizing it. The perl script consolidated raw EDI(1) files coming into some ancient HP-UX machine, then exported it...to an excel file, MS Access DB, and a SAP DB so that the accountants and the manufacturers could simultaneously get work processing the order. When I started people were inputting these all manually.
1: https://en.wikipedia.org/wiki/Electronic_data_interchange
Most people working in software understand this as a huge chunk of software projects are "let's get these people off of their spreadsheets!"