A lot of older folks play mafia and are used to old design.
Also infinite scrolling is intimidating on threads with hundreds of posts. It blurs all the posts into a never ending stream. It makes harder to navigate to specific posts you are not sure the location of. With pages you could roughly guesstimate what page the post occurred on based on maybe a 4-6 page range you think it can be on, and you can find it a bit faster because you vaguely remember it's either around top, middle, or bottom of page. With infinite scrolling, if it's post 367 out of 500+ you prob going to aim for the 300's ballpark. You better hope you don't tirelessly scroll past it or you either need to check 100 posts again or start doubting whether it's really a 300's post.
Also is there a way to filter by all posts a user made in a thread? There was this feature on old forums, albeit via crafted url.
It would also help if you can limit post search to a specific thread in addition to user.