A major question throughout the entire Marvel Cinematic Universe saga has been why Thanos waited so long before he started gathering and using the Infinity Stones himself.
The most compelling argument is that he wanted to wait until he knew where the Soul Stone was before alerting the entire niverse of his intention and risking everyone allying aginst him.
Which is why he lets others wield the stones in his possession, like Loki with the Space Stone or Ronan with the Power Stone. When he does, finally, decide to make his move, the Mad Titan makes the extremely not-mad decision to acquire the Power Stone first.
Equally importantly, why did none of the poweful heroes who had possession of the other stones try and turn them on Thanos?
A theory on Reddit from Lol-Guru explains the unique strength of the Power Stone if used against any of the others.
“Why didn’t Dr. Strange use the Time Stone when they were battling against Thanos?
“An answer became clear when I remembered certain key details from the Dr. Strange movie; even with the power to slow, pause, or reverse time we have seen foes with the ability to ignore the Time Stone’s effects (Kaecilius and his acolytes).”
This theory makes perfect sense.
If the remaining Avengers do as expected and start to try and collect the Infinity Stones from the past, they should make sure they have the Power Stone before they try and use any of them against Thanos.
FULL THEORY ON REDDIT
AVENGERS 4 IS OUT ON APRIL 26, 2019