I don't think that's necessarily fair. I agree that you're potentially dealing with 'the most casual' audience in many ways but that doesn't always translate to how people focus on the game itself. Or rather, you may have a shit tonne of people who may only buy FIFA and maybe CoD year on year, but many of those people take football gameplay really seriously. The license for FIFA plays a big part, the popularity plays a big part, but that doesn't always account for the significant shift from PES to FIFA.
Basing it on the experience of my group of friends - some of which play games a lot, some of which own a console pretty much just for one football game to play with mates - people switched from PES to FIFA because of the mechanics. I always considered FIFA to be flashier but mechanically a lot worse than PES and that changed - they matched the licenses and glitz with much improved gameplay. You're painting the market who play FIFA as people who simply buy based on what's the most popular - and while that's definitely a factor for many - that's not as widespread as you're making out, I think. PES may be coming back (I really, really hope it's back on form) but over the last few years, a lot of people who love football games and loved PES switched to FIFA for a reason and it's not just based on populism. (Edit: Speaking personally, I like the licenses but stayed away from FIFA because the gameplay sucked. Once it didn't, I nervously switched to FIFA and haven't gone back to PES for about 3 years).
I guess the short version of what I'm saying is that while a load of people who buy FIFA are not necessarily people who are more broadly involved in videogames, a lot of those people are very serious about their football games and don't purchase purely on the basis of popularity. I agree FIFA is a juggernaut, but if PES is legitimately a lot better in gameplay terms and the reviews/social media impressions from the demo etc. point to that, then I'd say you will see a noticeable swing. Sure, maybe most won't, but I know plenty of people who aren't "hardcore gamers" or whatever you wanna call them who will switch back to PES in a second if it's shown to be demonstratively better in '18.
