Google: They have an advantage in search that I think would be hard for a new entrant to overcome (mostly because of the amount of data they've collected). I don't think their advantage is algorithmic at this point so much as lots of data + they're a verb. Uber is practically a verb at this point, but I think their data is less valuable. I guess it would be comparable to Foursquare's data, but much more of it.
Apple: The whole industry has become a race to the bottom, except for Apple which has built a phenomenal brand (also see Nike for example of a valuable brand). I don't think the Uber brand is worth all that much.
SpaceX: I think it's fair to say setting up a rocket company has significantly higher barriers to entry than building a taxi dispatching app.
But Groupon I think fits:
Let's say that's true; the folks at Groupon are smart and they'll figure out online coupons. Why does anyone think Groupon will be able to maintain an advantage with that? What's to stop anyone from hiring some web developers and opening their own daily deals website? How does the whole industry not become a race to the bottom? Hell, given the price wars going on now, I'd argue it already has.
In the case of Uber, I'm arguing that the barrier to entry for other companies will be buying several cars in a retail market, because various manufacturers will be selling them.