Based on what I've heard from former engineers there, there are still plenty of opportunities for things to go very wrong. Maybe not this week or this year, but the sheer amount of lost knowledge means they're really skating on thin ice right now.

If they were to have a seriously catastrophic incident, it's conceivable that they might not have anyone left who knows how to get things back online. It's not inconceivable that they could eventually figure it out, but without that essential knowledge, who knows how long that could take? I imagine it would be pretty bad for Twitter's future if the site were unavailable for days or weeks on end.