They aren't gonna have a raubahn ex situation again because they learned their lesson. Shadowbringers avoided that scenario by splitting the beginning of the story between two completely different zones and not having any solo instances early on. This eased the server traffic and spread out how many people were running solo instances compared to Stormblood where everyone hit the first solo instance literally all at the same time.

I can see them having to figure out how to manage server traffic in the coming months, but they got time before Endwalker. Also another thing to consider is that many of the people flooding into the game currently are going to put it down before reaching the end of Shadowbringers. Statistically, most people don't finish the games they buy, so it's likely many of the current flood of newbies will either get bored or have their interest caught by any of the other many games coming out and put the game down.