1) i'm not. it's a one time fee.
2) the overwhelming majority of people will NEVER buy a domain name to host their email address or their nostr address. they just want to click a button and pay for a service that gives them what they want.
Discussion
If you make it the only way not to get impersonated on Nostr, they will. Your reputation means more than $5 a year.
You can't impersonate me. Only one derekross@nostrplebs.com will ever exist. Ever. Now, some clients don't check the validity of the Nostr address by calling the JSON to verify the ID matches the public key. However, they show an invalid indicator.
What is stopping someone from registering derekross.com and pointing your npub to derekross@derekross.com
Someone could, but that's not the Nostr address that I've been using. My identity is derekross@nostrplebs.com. if you get a new email address, you email everyone and tell them hey this is my new email address. The same goes for your Nostr address. You'd post and say hey this is my new Nostr address. If it's not from the same npub that you've been following them you'd be suspicious. This would be harder for new users that were looking for the correct Derek Ross to follow, but a little bit of work helps to alleviate pain points.