> Specifically, is there any good evidence to believe that convenience is making humanity weaker? [...] Do we even have a clear definition of "strong" or "weak"?
This, exactly. Unless we're talking about how much weight a person can lift, there's no objective definition of "strong" or "weak" person. "Strength" in this context isn't a property of a person at all, in my opinion, but of how other people perceive that person behavior - and that perception is so dependent on observers' life experiences, culture, religion, education and many other factors that it's practically meaningless. As such, the very notion of "strong person" is exploited in all kind of propaganda - with a concept this fuzzy it's easy to bend it to mean whatever you want it to mean.
"Convenience is making us weak" or "hardships make people strong" are examples of such propaganda in action. There's no meaning to them other than what the person talking wants them to mean (and, possibly, what the audience wants to hear). You could reformulate the statements to be easier to prove or falsify, but then the entirety of modern psychology and cognitive science would be against you, which is why nobody attempts to do this.
Well, that's how thought-terminating cliches generally are, so no surprise here. What's amazing to me, though, is that we're this deep into the thread and we still talk about them...