It's an asset from the point of view of the bank or institution that made the loan. And they can transfer those assets from one investor to another. And the value of those assets can be based on unrealistic expectations about future default rates, so in that sense, student loans certainly can be a "bubble".

The real damage done to the economy by the mortgage bubble wasn't all of the people who lost money on homes when prices collapse. The most damage was due to the effect it had on banks' balance sheets when their assets (the value of debt-backed securities) lost value, which meant they had to call loans and restrict credit to make up for the difference. In that sense, a bursting of the student loan "bubble" could also have damaging effects on the economy.

When there's a decrease in the loans going out, there's a decrease on the payments coming in at a future date.

When there's a decrease in the loans going out, there's a decrease in the demand for education services causing a reduction in the quantity available.

Repossession isn't a necessary condition for something to be in a bubble. Pulling demand forward and using laws/regulations/programs to funnel money into certain parts of the economy is what causes bubbles.

And, as has been discussed in another area of this thread, repossession of a college degree is possible. I'm not advocating it, but it would have a definite effect on incentives on both sides when making decisions about which college to attend, what degree to study, how much debt to borrow, and whether or not to repay it.

If student loan debt were bankrupt-able, and the degree revoke-able, I can see how that might remove some current problems (not that it wouldn't introduce others).

Exactly. Plus you can change the ground rules. In this case it can apparently even be done without changing the law :

Student loans are not excluded from discharge if excluding the debt would "impose an undue hardship on the debtor and the debtor's dependents".

So it only requires a small attitude change of judges for the bank's assumption that non-repayment is impossible to blow up. Given the attitude changes towards student debt that are occurring in the papers, can this really be that far behind ?

who says assets have to be transferrable. Is life insurance not an asset, even though, obviously, it's not transferrable?

Life insurance is transferable. Frequently lost in divorce settlements. Think of it as a Life-Default Swap.

Life insurance can be collected upon. How does one cash in a degree?

You can't directly, but a lot of people hope to get a reasonable rate of return on their degree, career-wise.