I'm sure the pay is great, but it's not a place for smart people who are interested in doing something. I've followed Francois (and had the chance to correspond with him a bit) for many years now, and I wouldn't be surprised if the desire to create something became more important than the comfort of Google.
I've been in tech since the 90s. The only reason I'd go is to network and build a team to do a mass exodus with and that's literally it.
I don't actually care about working on a product I have exactly zero executive control over.
For one thing as soon as you start doing your own thing you will quickly find your day eaten up by a trillion of small little admin (filling reports, chasing clients for payments, setting up a business address) things that you didn't know even exist. And that is not even taking into consideration the buisness development side of thing (going to marketing/sales meeting, arranging calls, finding product/market fit!, recruiting, setting up payroll....) At google you can have a career where 90% of the time you are basically just hacking.
At a 3 < n < 100 employee start up you absolutely are not "eaten up by a trillion small admin" and at the same time you can visibly see your impact on the product and company in basically real time. I've had work I've finished on a Monday directly lead to a potential major contract by Friday. I've seen features I've implemented show up in a pitch deck that directly lead to the next round of funding. Every single person on the team can personally point to something that they've done that has lead to our team's success so far. It's immensely rewarding to see a company grow and realize that without you personally, that growth wouldn't have happened in the way it did.
"90% of the time you are basically just hacking" is sounds fun, but I personally find it much more rewarding to see each week's work making incremental but visible changes not only in the product but the company itself.