Although tanin gives an unpleasant astringent effect in beers, in wines - also, but - it is known as antioxidant and protector against UV light. So, in beers can tanins also be welcome at least considering aging ? Or hops react to O2 and light more quickly than tanins ?
You're absolutely right that tannins (a kind of polyphenol) are anti-oxidative and that increased levels in finished have been linked to a lower degree of beer staling (in this paper, for example). I've never heard the claim of their anti-UV-light effects though, so I won't touch on that.
It's a tricky question because there will always be a trade-off between the increasingly unpleasant flavor of tannins and their protective power. Sulfur dioxide (the smell of matches) is the exact same way in that it acts as an anti-oxidant, but high levels are often unpleasant to taste.
Polyphenols are also highly reactive with proteins and in mashing, boiling, fermentation and conditioning, will tend to bind with them and precipitate (or form haze). This is largely what hot break and cold break is. So, mashing to increase tannins might not actually increase the amount in the finished beer, it may simply serve to help precipitate more protein during the brewing process.
I would also think that trying to provide tannins as a source of protection for aging beer would only be useful if you've already done everything you can to exclude oxygen, the real culprit for staling, from your beer (as they say, grab the low-hanging fruit before you reach for the ladder).
So, your theory is entirely sound (tannins will protect beer from staling), it's just important to remember that it will always be a compromise between flavor and stability.