There haven't been many straight forward deaths yet for such major characters. Muzan AND Gonpachiro dying so straight away with nothing else just like that is not going to fly. The last thing we saw of Tanjiro "alive" is being absorbed by the baby. You don't do stuff like that because you feel like it over what is meant to be the last boss.
That scene in itself is going to have some repercusion. Whether there's a fight inside of Tanjiro for between him and Muzan to take over his body or whether Muzan straight up took over his body, or if Tanjiro is now the main demon and a demon slayer, it just doesn't make sense to me from a writing perspective to have shit like that and it not being Chekhovs gun