None of the things you listed are suitable replacements for Excel. None.
It has nothing to do with existing files/compatibility. Excel is unparalleled.
The solution is actually just not using Excel. If you're essentially using Excel as a LOB backend and database, that should probably not be in Excel.
It's fine if you have a few formulas. As soon as you're busting out macros it's time to sunset the workbook and make an application. There's a lot of God Excel workbooks sitting around on share drives with no audibility or quality control.
Have to disagree. It depends on what you are doing. That the alternatives can be replacements, including the open source ones, is relative and should be looked at as a percentage.
If you listed out all the things that Excel can do, we might find that the alternative is at 80% or so (just a number), with some additional things that Excel can't do. That 80% could be good enough to switch. It should not be looked at as "all or nothing", especially for every person or business.
Can it open Quattro Pro files?
A whole lot of accountants/bookkeepers (including I) will totally disagree with you. LibreOffice is very good.. and not just Calc either. Writing a novel in Writer is a pleasure.
More people should use and contribute to LibreOffice!
That depends on your workload. I've been using Excel since 1993, and I find the things I've listed help me get things done just as well as Excel does, unless I have a pile of macros and vbscript I need to interop with.
This is not theoretical; I learned it by needing to get shit done in a context where having an activated copy of Excel wasn't practical. Excel was paralleled and in one case surpassed.