Well, challenging the debt on legal grounds shouldn't negatively impact your FICO score.
Presumably also if enough people broke this system, debt collection would be forced to change in a way that made it harder for people to do this. It is perhaps better if people who can manage to pay it do, so people who can't retain the ability to exploit the bad recordkeeping in the system as it is today?
Eventually your challenge (assuming you win) will have any negatives removed from your FICO score, but it may remain for a time while the challenge is in process and during that time you don't have the high score. Worse the lower score may mean other changes to your behavior that affect your credit indirectly - if you don't take out a loan you would have for example.
Presumably also if enough people broke this system, debt collection would be forced to change in a way that made it harder for people to do this. It is perhaps better if people who can manage to pay it do, so people who can't retain the ability to exploit the bad recordkeeping in the system as it is today?