Very true but depending on what latitude you live at, it may be impossible to get enough on sunlight alone. There's dermatologist that claims a link between latitude and mortality:

https://www.ted.com/talks/richard_weller_could_the_sun_be_go...

Recommended watching - there is more to sunlight than Vitamin-d.

That isn't so easy. I live in Norway. Where I am, winter days are only 4-5 hours long and the sun isn't all that strong. One can easily work through the sunlight.

In addition, you should be out with some skin exposed - more than just your face - for at least 15 minutes a day during daylight hours. In some areas, this makes it somewhat prohibitive during winter and you still might need supplements.