Hey hey, I hope this is the correct thread for this question:
I was wondering about how MANDATORY the traditional offense-roles are in modern Balance teams? Do you need a Wallbreaker, a setup Sweeper and a Revenge Killer/Scarfer on that archetype? And if not, do you need to make up for those in other ways? Idk, out of the blue, I guess hazard stacking could replace breakers? TWave on one or multiple mons or priority could replace Scarfers, etc. etc. etc.
Am I completely wrong about this idea? ^^
The way the metagame has evolved it's not super beneficial to think about roles in this way anymore. On a surface level, yes, every team should have a breaker, a wincon and probably multiple forms of speed control. But power creep, speed creep, and centralizing pokemon have made it so that this isn't the best way to view things, and you really must know the meta to build properly for it. Not just like, oh i know that zygarde was garbage last gen and is really good this one, but like which zygarde sets are in circulation right now, and how can I build a team that doesn't lose to them?
The way to build teams these days isn't by fulfilling roles necessarily; it's by covering metagame threats. There are too many, and they are too easy to lose to (or win with), so if you say 'landorus is my scarfer i've got speed control,' prepare to lose to ash gren. Or if you say 'hoopa is my breaker it's stupidly strong i'll break everything,' prepare to lose to magearna. These are contrived exagerations to some degree, but they are fairly accurate pictures of how you need to think now.
But like something you mention, hazard stack, for example, is a fine way of thinking still. The builds that are focused on a specific strategy that becomes a wincon are still good. It's just the traditional roles are less meaningful on a team level. That's not to say wallbreakers, sweepers, and scarfers, etc, aren't relevant; on the contrary these individual pokemon roles are just the same as always, but on a team level, you can't say 'i've got a breaker sweeper and scarfer, i'm good,' just because the number and diversity of threats don't care about your roles, they only care if you have an answer to them.
I hope this is helpful in some way. The easiest way to find out what you need to be preparing for is to hit the ladder, cause it's full of all kinds of stupid threats right now.
As a direct answer to the last part, twave is quite a bad move now (lost 10% acc and speed reduction is only 50% instead of 75%, meaning those threats likely still outspeed your walls), but yes multiple priority users can be a replacement for scarfers depending on team composition.