How the hell do you expect them to make money off a free-to-play game then?

The first suggestion, as you had already named it. The currency helps to create an artificial grind in many a "F2P" game. 

You could ask the same about AAA games, yet they make their mark from the £50 you're spending on the game alone. Why do they need to shove MT's into those games and rip them further apart?. 

Fortnite offers Skins and emotions for Microtransactions. What a grind do you mean? Everyone has the same character abilities in Fortnite BR (Save the World has not 200 million players).

B2P games with online component have MT (Overwatch is an example) because energy and bandwidth are not free. The money they gain from the 60 dollar pricetag is mostly to cover the development cost; but if they intend to keep, maintain and improve the service infrastructure for the next 10 or 20 years, they need to have MTs because at some point, population peak, very few newcomers are buying and entering Overwatch today, the revenue generated with the sells of the game to new players is minimal, it means they need a steady income from the current playerbase. Overwatch population actually peaked since last year, and now is declining, and I still don't know why they just don't turn it into F2P, it's the only way to bring new players to OW now.