I would guess there might be a few motivations for people to do this though, not necessarily just rudeness. Misplaced helpfulness, wanting to save face by not switching languages back, practicing English, who knows?
(What's really funny is when they finally give up and revert to French and then address a question to me - only for me to shrug with embarrassment and tell them I don't speak French.)
As a native French speaker travelling in Montreal, I often found it easier to talk English with French speaking people, to my great shame.
I recall a simple transaction with a cashier in a supermarket, and having her repeat the simplest things (like "hello") a couple time because I had no idea what she was saying or if she even was speaking French, this was awfully embarrassing.
Although I agree, Parisians are asshole.
I'd argue that there is a surprisingly homogeneous distribution of horrible people throughout the big cities of the world.
And at that time I had been travelling to lots of countries for years, and I never had any trouble communicating with people. Big cities, villages at mountain tops, desolated island towns. People were nice everywhere. Except for Paris.
If that has changed for the better recently then that's truly good news.