Quite a good point, Rob.
Of course, there are those that get excited about something new, and want to use it everywhere they can, but I think most Flexbox enthusiasts will be the first to say it is not for everything, and should only be used where it solves a problem better than the previous solution.
There is no question that floats and other positioning methods can do nearly everything you want to do, but there definitely are things that cannot be done with CSS, using a non-Flexbox method.
But in the end, developers should do what is best for their clients and their clients’ sites.
Cheers,
Atg