It's walkable and affordable because it makes decent use of vertical space.
I know a prominent member of the local government personally, and we've discussed this problem at some length. The fundamental trouble- and hypocrisy IMO- is that many of the people in the town bitch and moan when prices go up and interesting local businesses/artists/features have to leave. But these same people refuse to allow greater density and constantly try to also push the students out.
I'm fine with Google moving in to town if we can get smaller, more affordable units in greater number. However, due to the planning decisions being pushed by various groups, there aren't any studio sized apartments being allow to develop in the city, and the affordable neighborhoods are often 'historic', turning into student ghettos.
Most of my friends are high payed tech folk and don't have trouble with rent. But many others, like myself, are designers, photographers, writers and so forth. We don't have the money to rent a 2 bedroom apartment for ourselves, or buy a condo to sublet. The only other option at this point is to live with 3-5 roommates in a house, for 600+ a month.
I would LOVE LOVE LOVE to have an reasonable, clean, modern 1 bedroom studio like I had in Copenhagen. But that just doesn't exist in Boulder, and most attempts to make such things are blocked.
In the end, I think Boulder could address the housing and cultural concerns while still preserving the Open Space around the town. But we'll HAVE to accept greater density, better transit (which needs density) and the fact that sometimes in a city you'll see things you dislike. Without change on these fronts, I suspect we'll quickly go the unfortunate way of Silicon Valley...
If a place to too affordable, it likely suffers from major issues that make it undesirable (lack of employment, pollution, etc).
But, assuming you value some level of diversity, a certain level of affordability is required to maintain that diversity.
If you intend to stay where you are for the long term, you want your property value to stay exactly where it is. If your valuation rises, you might be forced to move by taxes. If it drops, you lose services and the place starts to get grubby.
Hail the direct democracy of CA in general and Prop 13 in particular!
Or, if you live in SF, Seattle or Portland, your taxes go up but you still get poor quality services and everything is still grubby.
What YokoZar was describing wasn't that, though. YokoZar was describing a way to turn a profit on increasing property values.
NIMBY fascists whine about property taxes because they want things both ways (see: Prop 13). That said, they usually benefit when property prices go up. Let's say that you bought a $300k house and it went up to $500k, and that your property tax rate is 1.25% (that's fairly high for the US). You're up $200k in home equity, but paying $2500 more annually in taxes. Even though interest rates are low now, you can get 2% on a 5-year CD, and possibly much higher rates in bonds or stocks. So, you come out ahead.
Obviously, NIMBYs don't like to publicize the fact that they're actually making bank, because people would hate them more than they already do.
A valuation drop, on the other hand, is unambiguously painful for an owner. Many of the foreclosures in 2008-10 came from people realizing that they owed more on a house than the thing was worth, so they walked away.