Thanos' arrival in introducing time travel to the MCU without breaking the whole timeline. One key element to make this all work was the Pym Particles, which not only give Ant-Man and the Wasp their size-shifting abilities but also act as the fuel of the MCU's time travel.
The Avengers could have borrowed the six Infinity Stones to reverse the Snap without a problem if it weren't for one oversight. 2014 Nebula managed to impersonate her future counterpart, steal a vial of Pym Particles, and infiltrate the Avengers' headquarters to make way for Thanos. The only remaining question was how Thanos was able to transport his ship and his whole army with only one vial of Pym Particles, but that can be explained with Ebony Maw, who probably reverse-engineered them offscreen.
Ebony Maw's reverse-engineered Pym Particles is the MCU's most brilliant minds, just showed up in 2023 with a sword and an army of alien dogs like he didn’t just come up with a revolutionary copy of Hank Pym's greatest invention.
Why Didn't Thanos Use The Pym Particles In Endgame?
The reason why Thanos didn't bring an army of size-shifting aliens with him may be simply that he would have been too difficult to defeat. This weapon would have become a distraction from the emotional weight of the Infinity Saga's climax, and it could have easily added another hour to the movie's runtime. Avengers: Endgame already had many different elements to juggle, and it needed to devote enough time to each one of them in order to do justice to the characters' journeys.
Directors Joe and Anthony Russo delivered a solid story, though in hindsight, it's possible to see some plot holes in Avengers: Endgame. As hot-headed as he is in Avengers: Endgame, it's still uncharacteristic for 2014 Thanos to dive into battle so unprepared. He didn’t take advantage of the Pym Particles, or the fact that he could have perfected his plans for years while his destination in the timeline patiently waited for him. In fact, he didn't even bring vehicles or ranged weapons of any kind. The Avengers were somehow more prepared than him, and they barely got to regroup mere moments after being brought back to life.
Considering how Pym Particles can make anything larger, Thanos could have also experimented on his version of the particles to multiply resources in Thanos' universe. The MCU hasn't been consistent with the logic behind Hank Pym's technology, but given how efficient Thanos and Ebony Maw were with their knockoff, it's possible that they could have come up with a version that let them increase the amount of food and water, thus fulfilling Thanos’ dream to be a righteous savior. Unlike the option of using the Infinity Stones to double all resources in the universe, the Pym Particles could have been a more reasonable tool to combat the effects of overpopulation.
There are way too many mistakes the Mad Titan had to make to ensure he had a cinematic battle with the Avengers. While it would have made sense for Thanos to stomp on the Avengers himself, his size-shifting abilities would have been an addition that Avengers: Endgame simply didn't need. It's likely that all of Thanos' oversights were an essential part of the one universe Doctor Strange chose to pursue.
