You may not be able to afford an attorney willing or able to face down David Boies or whatever landshark the publisher suing you hired. Settling may be your only feasible option.
Let’s say you run a small business making 250k/year. If you sue, it will distract you enough that you’ll likely make 100k less over the 2 years it takes. Also, the suit will cost you 50k and that won’t be recoverable even if you win. If you do win, you recover 25k in damages.
All of these numbers are plausible and demonstrate how skewed the decision to sue must be. I’ve been quoted 50k by an IP lawyer as the estimated cost to take something like this to trial, and they thought I might not win if the company is able to claim my sales aren’t covered by warranty.