There are so many things wrong with the idea of a "bubble" popping. People can't walk away from student loan debt, like they can walk away from real estate debt. Interest rates are expected to rise, but only slightly with the 90-day T-bill yield. Not the insane rates 20% rates that banks were hitting people with in the subprime crisis.
People may find diminishing returns for college vs the rising costs, but that isn't the same thing as banks suckering people in with ARMs and then using leverage to make huge speculations on CDO's that lost 90% of their value.
Here's a madeup example from the subprime crisis. Someone puts $0 down and buys a $500k home with a loan from the bank. The bank expects to make $500k in interest over the lifetime of the loan due to ballooning interest. Now, let's say variable interest rates from the loan kicks in, and the person can no longer make his monthly payments.
In the past, when the real estate market was good, he could just sell the house for $600k, and keep a profit. But, lets say now the price of the home dropped to $300k. There is no way you are going to expect someone who put $0 down on a home he bought 2 years ago to cover $200k in debt for a home he doesn't even own any more due to foreclosure. So, now, instead of having something worth $500k in profit, the bank owns something that is a $200k loss.
Banks lost over a trillion dollars worth of assets in a matter of weeks.
Look at what was happening for years before the crisis though. In those times, it was very likely that a home could increase in value from $500k to $600k in a few years. How was that happening? A lot of it was because people figured out that they could put $10k down, and do this to make $100k, or 10x their investment in a few years. The true speculators are the people who just bought real estate only because they thought the price would keep going up. This in turn drives up the price without being tied to any kind of intrinsic value.
You simply don't have that kind of pure speculation in the college loan industry because you can't just buy and sell college degrees.
Also, people aren't just going to declare bankruptcy from their student loans in the same way as the subprime crisis because they legally can't. Wages can be garnished from student loans, and it's extremely hard to have the debt erased due to bankruptcy.
The "bubble" scenario is sensational, because everyone sees a trillion dollars evaporate in a few weeks. Instead, in the case of student loans, I think that we may see something just as bad for the economy, but it will happen over years, not weeks.
In a lot of ways, I think that's why this problem may be worse, because it may be just as bad, but much harder to notice than the subprime crisis. Journalists think that the only sign of economic failure is when a "bubble" pops though, which just isn't true. Slowed growth over 20 years could be just as bad if not worse.