The Pixel isn't locked to Fi. It can be used on any carrier. Fi is also pay as you go which means you can leave at any time. It's really not risky at all even if they shut it down tomorrow.
I think the parent was stating that Pixel phones will soon be the only way to actually get into Fi. For now, you can start service if you bring a 6p or 5x but who knows how long they'll support that.
