Water resistance, or lack of, is the biggest worry when wearing an old watch on a daily basis. That's the bad news, but the good news is that a sufficient degree of water resistance can be restored in the majority of cases. Crowns can be replaced or new seals fitted, caseback seals can be replaced and a leaking crystal can be replaced too. Acrylic crystals can sometimes shrink slightly with age and this will compromise water resistance.
Let's be clear, if a watch is fogging up it's getting water in and needs attention, unlike grazed knees they don't heal up and fix themselves, I certainly don't advocate using a watch in this condition.
As for the movement, a watch that lacks lubrication will incur wear at a faster rate and invariably it's the self-winding parts that suffer, closely followed by the mainspring barrel walls.
Refinishing acrylic crystals isn`t difficult, I`ve detailed how to do it many times using fine grade wet and dry paper followed by metal polish and finally polywatch. A small hard rubber block and some 3000, 5000 and 7000 grit wet and dry paper, a tube of Autosol, a tube of polywatch, plus 20 minutes of your time is all that's needed.
I wear vintage watches most days, they keep excellent time and don't leak because I`ve sorted them out, and once they're sorted they generally stay sorted for a good few years.