I thought Dallas overachieved last year, with a lot of fungible guys playing over their heads. This year appears to be regression to the mean for some of those guys. Also, Josh Richardson has been pretty awful to start the season, which hurts because he should probably be their second-best player with as much time as Porzingis misses.
I do have something to say about Doncic though... and this also applies to Tatum. Lebron was drafted in 2003, won his first championship in 2012. Kevin Durant was drafted in 2007, won in 2017. Stephen Curry was drafted in 2009, won in 2015. KG, Dirk, Shaq, Hakeem, MJ... all of these guys took many years to finally get over the hump. On the other side, I can only think of one guy in the last 30 years - Tim Duncan - who came into the league and won a championship in his first few years as the best player on his team (and having a HoF sidekick, albeit an aging one, certainly helped). Kobe is sort of a gray area, because while he won fairly early in his career, he wasn't the best player on those Lakers teams - Shaq was.
Yet we expect guys like Doncic and Tatum to be ready to drag teams to championships out of the cradle. It almost never works like that. Strap in, because it's likely to be a long journey. This is why I'm not crazy about most of these all-in trades that want to sell out for a chance to win this year. Yes, we need to improve the roster... but we also need to improve the Tatum, lol. And there's no shortcut for that.
EDIT: Dwyane Wade is another guy who won pretty early. Like Duncan, he had an aging-but-still-excellent HoF sidekick to help him carry the load. Anybody got one of those lying around?