The other big issue here is the negative amortization of loans. As you described the numbers, you borrow 100K, make 60K of payments, but still owe 80K at the END of 10 years. Because the IBR payments are lower than payments would be on a normal payback plan, you aren't reducing the principal very much with your payments. So even after 10 years and 60K in payments, you are almost in the same place you started--you still owe 80K out of the 100K borrowed.
If anything goes wrong during the 10 years to jeopardize the loan forgiveness at the end of 10 years, then you will quite possibly regret having gone on IBR. For example, what if, after 9 years, you got a private sector job you couldn't refuse. At that point, you will almost be starting OVER on student loan repayment. Now you'd have to pay them in full--perhaps another 10 years but with double the monthly payments. Add it all up and your repayment plan will have caused you to pay back maybe more than double the principal amount. And that's not even considering the low salary during the first 9 years.
So, the plan sounds great, but the plan requires you to do something dangerous--underpay the interest on your student loans for 10 years. If there was no forgiveness at the end, you'd never want to underpay the interest, because it just increases your total payback amount significantly. So you're taking a gamble.
As someone said, the lower salary is a cost to you that offsets some or all (or more than all) of the loan gaming benefits. But, if we ignore that and focus just on the loans themselves, the possible outcomes are:
Best case: Successful loan gaming--60K in payments followed by 80K loan writeoff by government--so you borrowed 100K and only paid back 60K
Medium case: Pay them off yourself as quickly as possible--you borrow 100K and pay back 100K plus interest over 10 years (less interest if you try to pay back even faster)
Worst case: You go on IBR, but something causes you not to get the writeoff. In this case your total payback will be much higher than in the medium case.
So the question is: what is the risk that something could interfere with the writeoff? Possibilities include: high household income, inability to get and keep public sector job for 10 years, need to take time off, so you reach 10 years of employment after 13 calendar years (and pay 3 extra years of IBR interest), you take a private sector job, the government changes the program, etc. Those risks are non-trivial. Finally, I read somewhere (on a forum--don't know if it's true) that the lenders and/or the government do want to have the program to encourage people to do public sector work, but they aren't thrilled about paying off your loans for you. So, according to one forum post, there are various hoops to jump through along the way, and if you mess them up you can hurt yourself. An example might be having to send in a form each year reconfirming that your employer counts as public service. If the form gets lost one year, they could disqualify that year of employment from your 120 months count. If any of these things happen to derail you from the loan forgiveness, you'll wish you had paid the full payment each month instead of the IBR payment.