There's a healthy number of startups in Paris where English is the working language. It's true that day to day life may be more challenging without speaking any French, but probably less so in Paris than elsewhere

I disagree. I've heard this from people here, however after living here for some time, I believe the reality is that if you don't speak fluent French, you will be at a severe disadvantage.

I guess it really depends on the company culture, and one shouldn't over-generalize. I have non-French colleagues who don't speak French (and made barely any progress from their French lessons, but hey it's clearly not their priority ;) and are just fine. But where I work everyone naturally switch to English as soon as there's a non-French speaker in the room.

So my advice to new comers would be to pay attention to this when you visit for interviews. Do you feel reluctance, are people hesitant and searching for words when speaking English? Could be a sign they're not used to it. If they speak casually OTOH (even with strong accent and the odd mistakes) it's likely they're used to it: good.

This is my experience as well. You need to speak French.