Even with really powerful sunscreen, every second you're exposed some portion of those UV-B wavelengths are still getting through to catalyze the production of Vitamin D.
According to one study [0] on a sunny summer day in Spain you could have 25% of your skin exposed and only need 7 minutes for that day's vitamin-D needs. (Obviously longer in dimmer conditions and wintertime, but those aren't the times people are going to slather sunscreen everywhere either.)
Also, of course, there is a variability between people how much vitamin D they can produce, but we have no real clue about the causal connections there, but so far it seems that it’s way less important than other factors generally speaking, especially in the context of “many people”.
So I looked it up, like you should have. In the US it's 40%. You're telling me 40% of people don't go outside for a few minutes??
If 40% of people have a vitamin D deficiency, I think you should change your recommendation for everyone other than people that get a lot of sun.
Through this, I've learned that my Vit. D supplement works. My multivitamins work, and my B12 supplements work. My previous iron supplement was not working, and I was able to make a change to a different product. Then, six months later, a blood test showed my iron levels back in the normal range.