A company like Dropbox does not need to keep its headcount to maintain their current product and extract profits (of which they lose a big chunk to corporate income tax). If they wanted to show profit they can easily tighten, focus, and get rid of R&D and new product development.
They aren’t cash flow positive. No company chooses not to be cash flow positive to avoid taxes. That’s not how things work. Do you feel the same way about Uber and Tesla? Do you think they aren’t showing a profit to avoid taxes or because they still haven’t figured out how to sell their products at a cost that will cover fixed and variable costs?