Are .com's going to continue to rise in value as more people get online and there are only so many .com's etc.?
Or will the other .net, .org, and million other extensions dilute the prices on .com's?
What about in the next 5-25 years?
What else could come along to make .com domains drop in value?
To determine value, you first have to understand what drives value.
What drives value of any given .com domain name is need and demand
of that particular name.
You can own a name that might make domainers ooh and aah with amazement that falls flat when presented to every relevant industry entity. We've seen several of these make the rounds at the domainer auctions in the past few years. Go ahead. Try your hand with it at 30K or 50K. Try calling the CEO of Company X who you're just certain will collapse with desire and rush for his checkbook to hand you a blank... You won't be the first owner of (CategoryKiller).com he's talked to and told to piss off. Expensive lesson learned.
The strange thing with some domains is that while there may be no demand for them now, that could change in 24 hours, 24 days or 24 months as business plans change, CTOs change, new startups are formed, new web applications are built. Today's wallflower could be tomorrows belle of the ball.
Domains in general have definitely 'arrived' in terms of value. Don't let usual suspects- the asshole carnival barkers, used car salesmen, etc- tell you otherwise, about how there's infinite headroom and growth left to be had. There's always schmucks just like that crowing at the tail end of every growth industry, even as it's imploding... The idea that truly premium, definitive domain names are 'something with value' is no longer some novel, esoteric thing, even if a surprising number of idiots still insist on emailing with offensively low offers.
The game is a lot harder, but you can still make money. The easy money is all gone. You have to be smart now. There are a lot of dumbasses who succeeded early on who wouldn't stand a chance in hell if they had to start right now.