But anyway let's take this step by step shall we?
First of all there is usually verbiage in the contract clauses specifically allowing the debt to be sold on.
Secondly the clauses are in some jurisdictions mandated by statutory law
Thirdly now here's the fun part if the original lender and new buyer of the debt do not fulfil the contractual and/or statutory obligations in informing you then you do have legal recourse to block the sale of the debt or offer to pay off the debt in full early without penalty, again this is highly dependent on jurisdiction.
Finally you're bringing up their trainee debt. This has already been hashed out in this megathread AND prior megathreads. The original trainee debt AND in fact financing debt for the creation of Ador and in turn Newjeans was paid back in full by year 2.
In addition to this there is no actual trainee debt under any of the Hybe labels, it just isn't structured that way at all. There is no debt, no obligation that can be or should be held over their heads.
This was explained here RE: ADOR - HYBE Mega Thread - Chapter 10 || Summary on page 1
and here RE: ADOR - HYBE Mega Thread - Chapter 9 || Summary on page 1
and here RE: ADOR - HYBE Mega Thread - Chapter 9 || Summary on page 1
So in your hypothetical it's like you paid your mortgage off completely in the first year before they even had a chance to sell it on.
For the rest of what you say, that is up for the courts to decide but frankly I'm bored of people who say "I'm concerned about the other idols" if you were that concerned about the other idols then you'd be all for setting a precedent here and turning the tables against the agencies and putting the entire system on a more even basis.
That you're not...? Well I'm wondering why