I'll cop to "never" being an exaggeration.

The safety issue is a showstopper right now, and will be until computer control reaches the point where cars and aircraft are routinely driven by computer, and air traffic control is also done by computer. Not before mid-century for this.

Then you have the problem of millions -- hundreds of millions? -- of vehicles in the air travelling on independent journeys. That's a problem that completely dwarfs present-day air traffic control. More computers needed.

They are also going to be using phenomenal amounts of fuel. Leaving aside sci-fi dreams of convenient new physics, those Moller craft have to be putting at least 100kW into just hovering. (Back of envelope calculation based on 1 ton weight and 25 m/s downdraft velocity, and ignoring gasoline-to-whirling-fan losses.) Where's that coming from? Cold fusion?

"Never" turns into "not this century", by my estimate.

Of course, if civilisation falls instead, "never" really does mean never, at least, never by humans.