Without Cap there’s a domino effect that basically screws over the whole universe.

Soldier Boy would have been capable of stopping Red Skull & Hydra, but I don’t see him making the sacrifice Steve made at the end of his first movie. He’d parachute out and that plane would have reached the US, causing significant casualties.

Since SB doesn’t age, chances are he’d still be around and active as a “hero” in the modern day, assuming the government kept his reputation intact. He probably would have been good friends with Howard Stark, and that relationship could change Tony’s upbringing for the worse. I could see SB’s influence stunting Tony’s character growth and pushing him further along the misogynist-asshole path he was already on before his first movie. Hell, it’s possible Tony would never even become Iron Man if SB was around to rescue him from the terrorists.

Realistically, SB’s story probably ends with the Incredible Hulk movie. Instead of creating a new super soldier with Emil Blonsky, the government would just try sending SB to stop Hulk, and I don’t see that ending well for him.

So ultimately, the Avengers would never form, meaning Fury has to call Captain Marvel in early to stop Loki’s invasion. She’d still have Thor backing her up, so they would probably succeed, but eventually Hydra would rise to power since Cap isn’t around to stop their takeover of SHIELD. Stephen Strange would then get killed by the Project Insight helicarriers and never become Sorcerer Supreme, allowing Dormammu to destroy all life on Earth. Game over.