What are the benefits of publishing a Next.js version of the Drupal website?

These can be advantages (but it really depends on conditions):
1) Drupal is hidden (if you don't upgrade even because of security you are not in so big risk)
2) your servers can chill because load is usually much lower (typical usecase, but may differ)
3) hiring really good React developer is much much easier compared to hiring Drupal guy with great frontend knowledge (this is almost impossible now)

But yes, initial cost can be higher, you add complexity (which you can easily outsource)