Surprising decision. DNSSEC is important stuff, should be in the core of Plesk. With security developments evolving fast, it might get as important as secured HTTP is now on a relatively short term.
This certainly isn't a niche feature, but that's my opinion.
The same like having to pay extra for HTTPS or secured (web)mail access, or other security features the web require you to have.
Maybe it's better to implement a pay-as-you-grow model (based on the number of domains someone is hosting), but that's a different discussion.