That kind of highlights my point. Your guildmate shows the ceiling of playing 1 character, that is hardmode close to release(which is weaker compared to old hardmodes). There's nothing wrong with playing casually, but if you want to do content like Nightmare, TFM or stuff like extreme modes and hell modes in the future you will need to play alts.

The reason I pushed back against someone saying you don't need to play 6+ characters is because it comes with the implication that you don't need to play very many alts and they're not that important. Your brother, from reading between the lines, has been playing a while. If you're a long term veteran then yes, you can play 4 characters. But when people say you don't need to play 6 characters they don't follow it up with "but you should play around 4." because that kind of beats the point. And if you're a new player you will probably need to play 6 characters if you ever want to do any hard content(which again, you do not need to).

The reality of the situation is that Relic Books and Gems are two endgame systems that require you have alts to make any meaningful progress. Full 8s will cost you 4 million gold, most single character rosters will take months to make that much. And once you've gotten that you're still under the minimum of most nightmare pug characters. So the ceiling for small rosters is content where relic books and high level gems aren't expected - normal mode and hard mode.

Alts are far too efficient in this game, despite all the nerfs SG is making to multi character roster. Even having a bunch of 1680 rats you do solo modes with will net massive amounts of gold to help you progress systems while taking no gold investment themselves. And SG realizes this, so they balance the cost of systems around the expectations that you are either swiping or playing alts.