In the mental realm, the opposite may be true: the average person may be experiencing a pretty thorough mental workout just from day-to-day life.
Depending on what counts as a "mental workout", the direction of modernity's effect on one's mental challenges seems unclear to me. Many people have much more cognitively demanding jobs these days, but our social life has atomized so significantly that we no longer rely so heavily on being woven into the larger social fabric of a community, where we'd need to devote many more of our mental resources to the task of keeping track of the moods, sensitivities and interactions of the members of our social networks.
We also have historically unprecedented levels of security along many axes: access to food and health, protection from violence and property crime. Maternal mortality and the death of children is extremely rare, and we're able to be in instant communication with our loved ones anywhere in the world. And as FTPickle mentions in a sibling comment, we've basically eliminated the idea of boredom in modern society.
It may be that coping with the complexity of modern life has us all constantly doing mental push-ups, but I think it's at least plausible that we're a bunch of "mental couch potatoes" who, without realizing it, have given ourselves a very cushy mental landscape.
Oh sorry! Having recently been deep-diving on this when considering the process I forgot some people might be squeamish about it. I've put the detailed description under a spoiler block.
(Spoiler block contains description of eye surgery.)
You may be thinking of PRK? LASIK
involves cutting a flap in the outer layer, which is peeled back and they reshape the corneal stroma, then the flap is put back in place and the cut around it
heals. With PRK the outer layer is removed entirely.
Not that I'm saying you should get LASIK — I got it done recently and it's been marginally better than glasses or contacts, but it hasn't been life-changing in any way so far.
Plus it doesn't really solve your problem, since one of the nicest things about photochromic lenses is that you're already wearing glasses and they just kick in when it gets too bright. After LASIK, you have to carry around sunglasses, remember to put them on and not lose them! I've actually been considering getting non-prescription photochromic lenses to get back some of the nice "automatic activation" features of my old glasses.
To be clear, I don't have high confidence in the direction, much less the magnitude, of the effect here, and I do not have any personal experience of tight communities.
My thinking on this was from a general sense that humans devote a lot of our cognitive resources into social games — signalling, fashion, politics, mating, etc — and it's plausible that modern society allows us to opt out of a lot of the most challenging parts of these by interacting with institutions with formal rules rather than communities with shifting and informal rules.
I get the impression that it is much more mentally challenging to navigate high school than it is to navigate adulthood, and high school tends to be a much more tightly woven, "small band" community where the participants usually don't have the option to leave and join a different community. That said:
Great point! I also find this taxing, and the story you tell is also very plausible. Making friends as adults is famously difficult, and friendships do not seem durable in my circles. That said, I think for a lot of people the solution to this is to give up on trying to make close friendships, and to instead have a loose network of acquaintances who you see intermittently. This seems emotionally draining but not mentally taxing, since you have very little investment in these relationships.
On the other hand, it could be that the lack of a strong community means that all your other mental activities are more difficult (in the fitness analogy this might be wearing a weighted vest all the time). No one to confide in when you are having relationship troubles, no one to watch your kids when you need a break from child-rearing to accomplish something. Of course, in the modern world you can pay people money to do these things (therapists, babysitters) with no expectation of reciprocity, so again it's possible that we're able to use specialization to avoid what might otherwise be mentally challenging activities.
My overall point here is that I don't think it's obvious that we're living in a more mentally challenging world today, though it's also not obvious that we're living in a less challenging world, either. It might be possible to get a better estimate of the magnitude and/or direction by getting a better understanding of what the mental equivalent of exercise is, though, since the mental activities we do today are qualitatively different from the activities we did in the past.