Easier said than done. As a general principle, existing features introduce maintenance and create drag when adding new features, so they better be worth it.

It's easier to justify when you don't have customers, just data sources.

The reason to support .xls is that lots of other software besides Microsoft office use it and thus lots of people benefit from compatibility.

You can still import it, you just can't export to an especially old version of .xls.

Just do it like Microsoft products do: big warning that you could lose new features whenever you save to the old format.

I think this is a big mistake for Google.