I mean... I still have my Microsoft Surface RT (the very first Surface ever), and the battery hasn't degraded that much to my surprise. I'm pretty convinced that Microsoft Surfaces are extremely long-lasting devices. I even still see people using the Surface Pro 2, Surface 3, and Surface Pro 3 whenever I go to Starbucks, so that's a big indicator that these devices last quite a long time if you take care of them. :) Sorry for my late reply, by the way! 😅
No, not at all. It's 100% free to switch out of Windows 10 in S Mode. Not sure where you heard that information from, but just know that you can switch out of S Mode for free at anytime within about one year of having your Surface Go. So you can try S Mode out for a while to see if you like it (which is what I'm currently doing, myself), and if you don't like it you can switch out for free at anytime by visiting the Microsoft Store (the Windows app, not the actual physical store itself) to do that. I hope this helps!