>>most supplements are only helping you in some way if you’re deficient in what they provide

Depends on what you mean by 'deficient'. The MDRs are mostly about avoiding short-term deficiency disease. I buy Bruce Ames' 'triage theory' (if you have enough of a nutrient for use right now, but not enough to support long-term health, your body will use that nutrient for what it needs right now, shorting the long-term process).

On top of that, nutrition is an individual thing determined by our genetics. For instance, some of us are poor methylators, so we need methyl donors in our diet - but other people don't. (My first hint: high serum homocysteine in spite of getting more than the MDR of B6.)

As far as vitamin K goes, there are two variations (different side chain), and most people are probably deficient in K2 (we can make it from K1, available from chlorophyl-containing foods, but very few of us are good converters). K2 has nothing to do with good skin/hair, is more involved with osteocalcin and moving calcium from the serum and soft tissues to the bones. Osteocalcin is important for energy metabolism, so it could possibly give you more energy at work if your work involved something aerobic.

As far as ‘supplements for people over 40’, I've read clinical trials showing over-60s respond well to a combination of Glycine and N-Acetyl-Cysteine - and I've also seen trials showing that creatine can slow/reduce sarcopenia (muscle loss) in over-70s (summary here). We can produce our own creatine, but the process consumes methyl donors. As a sub-optimal methylator, I've been taking it (in much lower dose than muscle builders) just to reduce the methyl groups I need in my diet, plan to increase it eventually.

I’d also point out that researchers have been using omega-3 fatty acids to reduce serum triglyceride — high serum triglyceride with simultaneous high glucose seems to be how insulin resistance starts in skeletal muscle cells (check Gerald Shulman’s fMRI work showing the link between those and blocking GLUT4 receptors).

All that said, I don’t disagree with your point that people are being too credulous about the claims about supplements. But there is a role for at least some of them.

TL;DR: It's complicated... and there's a lot of individual variation. Also don’t believe the broad claims made about vitamins.