As I understand the plaintiff's argument, each renewal is a separate act of cybersquatting. Can anybody more familiar with anti-cybersquatting law clarify whether this is a valid argument?
Basically, no, since the laws deal with the intent of possessing the domain, so unless intent demonstrably changed from when it was first registered, this is just an attempt to shove a wedge into where the plaintiff thinks there may be a crack.
There are a few laws that can get involved, but the most important in the US is the Anticybersquatting Consumer Protection Act.
I haven't read it, but it appears the act lays out some guidelines, and among some defenses are "Registrant’s prior use of the domain name in connection with the bona fide offering of goods or services" which should apply here. The act also lists actions that would indicate bad faith by the domain holder, like "Registrant’s intent to divert customers from the mark owner’s online location" and "Registrant’s offer to transfer, sell, or otherwise assign the domain name to the mark owner or a third party for financial gain, without having used the mark in a legitimate site", but those don't appear to apply here. The fact the domain was renewed is not relevant to this law.
The other important "law" is by appealing to ICANN under the Uniform Domain Name Dispute Resolution Policy, which unfortunately has historically been more fruitful to trademark owners when going after people legitimately using a domain that contains a trademarked name since it allows a person to complain that "a domain name is identical or confusingly similar to a trademark or service mark in which the complainant has rights".