I agree that some businesses may be able to avoid paying for Excel Online and/or other Microsoft products. But once you get on their radar, they will contact you. Many years ago I was purchasing Microsoft products as needed (i.e. standalone copies of Office from BestBuy, Windows user licenses from Dell, etc.), rather than via Volume Licensing. We had been using Linux, OpenOffice, etc but had been gradually increasing our use of MS products.
Then I received a very polite email from Microsoft saying they were going to audit our company. Later I received a very large and complex Excel spreadsheet to fill out, which wouldn’t run on OpenOffice requiring me to purchase a copy of MSO for my computer .
Long story short, the back and forth with MS took 3 months but we passed with flying colors. Which is a good thing, because the fines can be staggering.