So if I default on a loan, I should not pay the collections agency, and after a few months(??) it will just disappear entirely, even off my credit report?
Your statement confused me.
You totally missed that credit school didn't you.
if you default on a "credit" tradeline, be it a loan, car payment, credit card, hospital bill, etc. the date of last activity (the last time you paid them something) is when the clock starts ticking. Something ike 90 days after 7 years from that point is the last day that tradeline can exist.
If you make a payment or pay off the debt - it resets that date. So if you found an old hospital bill from a broken arm when you were 16 on your credit report after 6.5 years, and simply pay the x-ray place $100 for this old bill that might have already been paid by insurance, what happens is now today you start over with a new collection account that says in the comments "paid". It lasts another 7 years (plus 90 days I don't recall why but there is some period of 90 days in here somewhere)
Credit items have a negative or positive effect. A chargeoff is pretty serious - a defaulted loan even more. But all things credit go down in effect with time.
So where you might have had a credit score of 720 before with a 6.5 yr old $100 collection - by paying it you make a brand new collection account (that says paid), with maximum negative effect. So now your credit score might be 680.
This is why it is counterproductive in a lot of cases to pay bad debt.