I think Microsoft Office's mere existence precludes Google ever being considered a monopoly in the office apps space.

Mere existence of a competitor doesn't preclude an anti-trust case for monopolization or abuse of a dominant market position.

Lots of office suites exist, but Microsoft has to tread lightly. In Recent News, the EU is dinging them for bundling/tying Teams with Office. Because it's a dominant market position, regardless of competitors.

If at some future time, Google Sheets has a significant market share, they might get dinged, too.