Any topic under 10k replies is no problem, except perhaps on fairly old Android devices from say 2013/2014. A topic with 100k+ replies is a disaster, unless you are absolutely sure all your users are on fast internet connections and are using desktops or laptops built in the last 5 years.
If you don’t mind the hosting fees in terms of the extra pageviews, storage, etc of long chat topics then Do As Thou Wilt.
Note that it’s not just the storage cost, the organizational cost, the bandwidth cost – it’s the mental cost of search being cluttered with a bunch of ephemeral chat BS basically forever.
As a data point, daily web chat is critical to Discourse as a distributed internet open source project with team members in 8 different time zones. We only “see” each other online, outside our yearly world meetups. We’ve switched internal chat providers 3 times so far in the life of the Discourse project. Every time we switched, we threw away all chat history with zero side effects whatsoever. Zero!