I suspect there's good reason why HN is still using this old codebase. YC after all is not short of the cash needed for a complete revamp.

The site is very much hacked together, but works... In a lot of ways, this reflects the hacker ethos of getting something up and running quickly at low cost while still producing value.

A revamp might have negative impact too by attracting a wider, more mainstream audience which could possibly dilute the purity of the community here.