For me, the death that should have happened was Happy Hogan in Iron Man 3. When I first saw it, I really thought he was dead in that scene with the theater explosion. It would have greatly raised the stakes for Tony even though we'd have missed out on seeing Favreau build on the character more in the Spider-Man movies and Endgame. I think there's always creative hesitation when it comes to killing off secondary characters and villains in the MCU, especially since it's even more prevalent now that they have room to grow down the line (Darcy in WandaVision being one such example), hence all the fakeout deaths we've seen over the years .
Happy Hogan was another I thought of too. His character didn’t really develop afterwards. It’s almost remained a mainly comedic role except for a couple of emotional parts after Stark’s death.
His role in Spider-Man Far From Home could’ve easily have been filled by Ned.
Had Fury died then it would’ve allowed Maria Hill to step up.
This is the thing about the deaths. It allows others to step up. It creates jeopardy, freshest things up, deals with characters that cannot be really developed (e.g. love interests to Hulk), leads to those darker motivations for our heroes like vengeance, and it gives opportunities for others to step up. I just can’t see a downside.
As to Black Widow, for me if the plan was to have these characters face off against Thanos and his kids then they all needed a boost.
Sure she had help, but surely it would’ve been easier and more plausible if she had been enhanced. All these enhancements take is one line or one flashback and we’re done.
Black Widow had to fight Proxima Midnight.
And "normal" humans surviving insane damage is par for the course with these types of movies. Even if you look at the Nolan Batman movies, (one of the most "grounded" CBM series), Bruce Wayne still survives a whole load of stuff in that trilogy that would either kill or permanently cripple a real-life person (falling dozens of storeys, being set on fire, getting stabbed and shot, getting his spine snapped and somehow escaping a nuke with a six-mile blast radius in only 5 seconds.)
That's a specific issue for Batman because there isn’t much scope to enhance him. The MCU embraces magic and gods, and enhanced human beings, so there are more options.
And my understanding is that in the comics both Iron Man and Black Widow have been enhanced at one time. So they could’ve done it.
My rule for superhero movies and movies generally is that the creators can make up whatever rules they like, but then they need to stick to those rules. So for example Superman is pretty much invisible which is pretty bring except for kryptonite which then creates the jeopardy.
The MCU made the decision to keep Iron Man and Black Widow as normal humans, and then decided to give them central roles figuring Thanos and his kids. I think they broke their own rules.