The Marvel Cinematic Universe (MCU) has always been interested in Infinity Stones since the very beginning. Yes, I mean it! Even Captain America was into stopping Red Skull from using the Tasseract before he became a “Capsicle” (pun intended). Captain Marvel had received her amazing powers with the exploding of an energy reactor directly connected to the Tasseract! And if someone doesn’t know at least the plot of Infinity War and Endgame, he or she has been living under a rock! I AM SERIOUS!!!!
So, what would happen if there were no Infinity stones.
The first thing that comes to our minds when we think of Infinity Stones are the words, or movies, “Infinity War” and “Endgame”. While it is true that these would not exist, it could also be true that these two movies will exist, just in a different way. Maybe Thanos would find a way to try to obtain the whole Universe and “Make it right”. A way, Marvel hasn’t introduced yet. The next thing that comes to our mind, is Red Skull. Red Skull would not have died and maybe Hydra wouldn’t have been formed if there were no Infinity Stones, because the main and primary aim of Hydra was to acquire all the Infinity Stones. Next we think of the Captain Marvel. As everyone knows (hopefully), Captain Marvel received her amazing powers by absorbing some of the energy of the Tasseract. This made her the most powerful Marvel character (my opinion). If there were no Infinity Stones, she would have been just a normal person and would not have gone to another planet, plus there would not have been a plane crash (from where she learned about the stone). In “The Avengers”, Loki had come to Earth to get the Tasseract and to conquer the planet. He would not have done that and maybe would just come to Earth to conquer it. I think that the whole web-series Loki, would not have taken place and he would not have been a variant. Coming back to Infinity War and Endgame, Natasha Romanoff (Black Widow), Heimdall, Loki, and specially Iron man, would not have died!
Vision literally would not have even existed! Which would technically mean the Wanda would not exist… Kind of…
Therefore, I think that Infinity Stones are and will always remain an important part of the MCU and is already the reason that so many heroes like Captain America and Captain Marvel would not even exist.
Edit: 1:50 pm, 13/9/2021 These Ideas were mentioned by @JSheelan from Fandom.com Thanks to him. These are my ideas which could happen at centuries ago if each stones doesn't exist: Celestial couldn't wipe civilization efficiently as there is no power stone. Dark Elves conquest could be stopped easily by BOR (Odin's father). Tesseract's acquisition could also be altered too. As Asgardians where the first ones held in its history till now. Master of mystic arts could have major alterations as their one of primary weapon or defense not exists. Nothing major is known about mind stone beginning. Many souls could not be sacrificed as soul stones doesn't exist. Well What if we remove stone in a particular movie alone (Removed at each individual movie points) : CTFA: Captain America exists, Hydra's has definitely major hindrance as tesseract powdered weapons or technology doesn't exist, they have to move to alternate sources. CM: So basically Carol doesn't turn into Captain Marvel, if Mar-vell doesn't find another source on her mission in Earth. IM1,2, Thor, TIH: I think it goes similar. Avengers 1: It has a drastic change as two stones doesn't exist at this movie. This could lead to not even forming of Avengers as there is no necessity and still in development phase. IM3, Antman, CTWS: No drastic change. TDW: If Aether doesn't exist drastic change happens, dated even before Odin itself. GOG1: Guardians doesn't form as power stone doesn't there. Avengers 2: Mind stone doesn't exist, so no Ultron, which leads to no Vision. Peter is not an quicksilver but Wanda is still a witch, as Mind stone only amplified her powers. CTCW, Thor Ragnarok, Spiderman Homecoming, Black Panther, GOG2, AAW: No drastic change. Doctor Strange: As mentioned before major alteration. Avengers infinity war, Endgame: Doesn't exist.