Yes, if there was a valid assignment of the debt and required notice, the debt is collectable. But, there may be a statute of limitations problem if the lender just sat on its rights and did not actively try to collect.
Yeah, they forget to do things like notice of sale. But hey, they are important people doing God's work and they shouldn't have to be bothered by the little things like following the law. Too big to fail, too big to jail. It's good to be a
Systemically Important Financial Institution.