If one actually watches, instead of making rhetorical arguments, they'll observe that there are different types of products, and one can't make a generalized statement.
Windows for example, is expanding their ad-base, even if they have a base revenue due to the product being sold, so it's reasonable to present a slippery slope argument.
Ubuntu is not (directly) sold, yet, they've removed the ads (Amazon sponsored searches).
Android products have different dynamics - differently from O/S, the cost of switch is considerably lower; users do complain/act when advertisement becomes too invasive, so there is a balancing force.
This balancing force applies also to messaging apps, so the slippery slope argument is not realistic. See the significant number of users who switched to Telegram when WhatsApp was sold to Facebook, or when they changed the privacy rules.
I don't know how many users expect to freeload messaging apps (that is, to expect that apps don't monetize metadata or ads or not to directly sell the product). I have heard people complainints about Whatsapp's privacy invasion rather than Telegram's advertising (as a matter of fact, I guess that none of the people I know has ever seen a Telegram ad).
In this specific case, paradoxically ads are less dangerous, because the cost to the user is overt, as opposed to metadata (ab)use, which is covert.
