That's vastly underestimating the hardware on other devices as well as overestimating the danger. There are plenty of software optimizations that can be done to enhance security that work even if the hardware isn't ideal. Simply ignoring those devices is letting perfect be the enemy of better. Not everyone wants or even can buy a pixel, and that just excludes all of those users (and also sends even more money Google's way, which I would honestly like to avoid).