That Downey Jr., the cast member who kicked this whole thing off in 2008's Iron Man, gets billed last is testament to Marvel's history of keeping audiences' butts in their seats by throwing in scenes after the movie technically is finished. Ever since Nick Fury told Tony Stark about the Avenger Initiative at the end of Iron Man, the studio has tacked on these little blips of plot. (Even the mostly forgettable The Incredible Hulk had a surprise Stark appearance.) In recent years the trend has moved toward multiple scenes breaking up the credit scroll. Some are totally extraneous -- for humor purposes only -- and others inch the narrative along slightly.
Marvel paved the way for post-credits scenes to become de rigueur in franchise (and sometimes even non-franchise) filmmaking. Get up before the end of the credits, and you risk missing an important plot point. (Even an art house film like Suspiria got in on the game.) That's why it's so striking that Endgame ends where it does; it makes you think that the narrative is actually complete, even though that's not at all the case.
The lack of an after-credits tease, emphasizes the ostensible finality of this era of the MCU. And here's where we get into actual spoiler territory so beware. By the end of the movie Tony Stark is dead, having sacrificed himself in order to defeat Thanos once and for all. An absurdly stacked assortment of cast members from various MCU movies -- among them Michelle Pfeiffer and Angela Bassett, two legends with no lines -- gather for his funeral. The one person missing? Natasha Romanoff a.k.a. Black Widow who fell to her death in pursuit of the Soul Stone, another selfless act. Meanwhile, Steve Rogers a.k.a. Captain America went back in time to restore the Infinity Stones and the natural order. Instead of quickly zipping back, he ended up living out his life with Peggy Carter. He returns to the present-day an old man and hands off his shield and title to Sam Wilson. Endgame is set up as a passing of the torch.
The one clip that does play when the movie ends is pure audio, the sound of hammer clanking away on metal. It's the sound Tony building his suit. While it would be easy to theorize based off that addition, it makes more sense as tribute. Tony started this whole franchise when he decided to become Iron Man, now he's gone, but his legacy will live on.