In most of the US, probably. It's not so clear cut in London. The tube is usually faster and more pleasant. (London's air quality is pretty bad, and many Uber drivers have an annoying habit of driving through heavy traffic with the windows open.)
Sure, but London has always had taxis, too, because sometimes they're the better tool. It's not like some people are taxi-takers and others are subway-takers. You use whatever's best at the time. When I lived in Boston I took the T every day -- but on some occasions, a taxi was the better choice.
And the negative externalities of all those additional cars on the road are far worse than if you just have more people using the existing subway, too.
If you're going to do a full accounting of all the costs of all those additional cars on the road, and all that additional congestion, then rideshare doesn't come out looking so rosy.