No, I've never found anything that specifically says that someone can become ineligible for IBR, but the federal government obviously is going to wipe your debt clean if you end up making six figures as an AUSA or something when you have only 50k in debt.
The way IBR works is that you never have to pay more than a certain percent of your salary each year to your loans. At the end of the time period, which should correspond with your loan period somehow, if you still owe loans then the federal ones are forgiven.
Say you have a 10 year repayment plan, and you graduate with a fed gov't job in hand. If you owe 200k in loans, you put it all on a ten year plan, making your payments (just a guess) $2,500 a month. If you start at 75k a year, you'll only have to pay 12% of that amount to your loans, like 10k, each year. You'll get out of 15k of payments that year. That is not immediately forgiven, it effectively gets chucked back into the amount you still owe. After 4 years, if you are making 120k, then you are paying 14,000 a year, and will not have to pay 11k that year. At the end of the 10 years with the gov, you'll still owe 100k in debt because your payments were only half of what you agreed to pay. if 75k of your original debt was federal, though, about 37k of that is gone, forgiven. You'll still owe 63k on your private loans. That may actually be higher because of interest, though I'm not sure.
At any rate, that is the equivalent of a 37k scholarship. Not amazing but not bad.
by my reading of it, if you leave the gov't after 9 years, you'll owe the whole thing, but at least your payments were manageable while you were in govt.
There is also a 25 year version, but 12% of your salary would likely cover your payments if you make over 100k in federal work.
I don't think IBR is a very compelling reason to stop considering scholarships.