I'm not complaining. I think that for the most part GOG's prices are very reasonable (especially when there's a sale), but I've noticed something strange: I've never seen the regular price (when it's not on sale) of any game actually reduce. Does this mean that games that cost $50 now, will still cost that in 15 years? If so, WHY?

It's not a common thing, but there have been many cases. The never appreciated enough mrkgnao is logging all price changes on GOG since April'15 here.

This is wild speculation, but I believe that because of the current (over)prevalence of sales on all platforms, the publishers actually have a incentive to rarely if ever drop the price: drooping the price on a sale to a more reasonable level is better if the original price is higher, because then you have a more attractive percentage drop for the eyes. The end result is, even at very high percentages, with a high non-discounted asking price you still have to pay a reasonable amount of money.

Oh, there are reductions. Maybe not so frequently, but there most certainly are. Witcher 2 used to cost $50 when it was first released here back in 2011, but during the next two years, its price dropped to $20. Witcher 3 used to cost $60 when it was first released, but some months ago, its price dropped to $50.

It works the way around too, sometimes publishers raise the price (see the old Interplay catalog which went from 5.99$ to 9.99$).

Here is a list of all the (83) games that had their full US prices change (mostly down, but some up), since MaGog began recording this (May 2014):
[url=http://www.an-ovel.com/cgi-bin/magog.cgi?ver=731&scp=gdspurio&dsp=ip&ord=&flt=prh~1~&opt=n]http://www.an-ovel.com/cgi-bin/magog.cgi?ver=731&scp=gdspurio&dsp=ip&ord=&flt=prh~1~&opt=n[/url]

Note: In case you're not familiar with MaGog's display, you want to look at the bottom of the rightmost column for each game row, for lines that read: "{On [Date], price changed from $xx.xx to $xx.xx}".

Prices change. GOG just doesn't inform you of it.

But MaGog does...

P.S. As already mentioned, if you're interested in keeping up to date, follow this thread (though most of it is about regional pricing):
https://www.gog.com/forum/general/good_news_price_updates

Well thanks. I guess I just never noticed it. You'd think they'd advertise when a price was reduced, in case it would cause more people to buy, wouldn't you?