Yesterday I seen someone claiming on Reddit that they stopped using mailing list in favour of using rss

Yes that works to read them - and I do use that.

But if you want to write a message in reply or start a new conversation - RSS does not allow that. (In the cases that I use RSS - I will just copy to my mail app - it is more cumbersome than just replying in mail (or usenet via gmane) but this is not something I do often for those lists.