Exactly. Once they get that first hit of sweet, sweet ad revenue, there will inevitably be pressure to grow it, and then grow it faster. Zoom does not strike me as the kind of company that will be able to resist the temptation to insert ever more obnoxious ads, as long as it leads to revenue growth. (That said, they don't have to support a free tier at all, and it's not free to run, so I don't completely begrudge them looking for ways to monetize it beyond just Pro upgrades).

I’m imagining a future update that increases their ad revenue by 10% due to a bug that randomly ends calls prematurely, thus creating more ad views.

Later on, a precocious junior engineer discovers the bug, but isn’t allowed to fix it due to the revenue hit.

Unless it's an open-source/self-hosted application, you're always at mercy of code owners.