Though at this time I prefer a 60s version, I'll be happy with the FF in the MCU regardless of what time period they happen to exist. But let's look at some of the concerns for folks who are strongly opposed to a retro version:
1) We're missing out on Pete and Johnny's friendship, Hulk vs. Thing and Reed outsmarting Tony!
Time travel is a thing, so we're not necessarily missing out on any of those meetings. But if the only reason for putting the FF in the modern day MCU is crossover potential, I would reconsider that line of thought. We don't know how much longer the Sony/Marvel agreement on Spidey will continue, or if Sony would allow Pete showing up in an FF film. Thor appears to have replaced Ben as Bruce's punching buddy, and we don't know what role either of these founding Avengers will take in the MCU post Infinity War. And I wouldn't be surprised if Stark is either retired or dead by the time the FF gets rebooted into the MCU.
2) Aliens weren't publicly acknowleged in the MCU until Thor came down to earth!
Well, we know from Guardians, Thor and AOS (I know, its TV) that aliens have been dabbling in earth since pre-history. So it could be entirely possible that the FF could have had run ins with Galactus ("Weather Balloon" - government scientist) and the Silver Surfer ("Experimental Aircraft" - government scientist) without being widely acknowleged. In the 60s there were no camera phones, internet, or recording devices posted on every major urban intersection. It would have been a lot easier to cover those things up back in the day.
1) The FF are high profile celebrity adventurers. Why hasn't anyone in the MCU mentioned them? Huh, why not? You can't explain it, can you?!
They have been mentioned, numerous times. But every conversation regarding the FF since 2008 has been cut from the film or TV show due to run times. All of these discussions of a team that existed before many of the MCU heroes were born will be revealed in the Marvel One Shot "Whatever Happened to the Fantastic Four?" Highlights include:
- The complete conversation between Fury and Stark at the end of Iron Man : "I'm here to talk to you about the Avengers Initiative. It's a superhero team, kinda like the FF back in the day. Remember them? Nah, you're probably too young. I think your father did some work with them back in the 60s."
- The complete conversation between Ross and Blonsky in The Incredible Hulk: "That thing is Banner. He's become incredibly powerful, like a bigger version of The Thing from back in the 60s. Remember him? Nah, you're probably too young. He was on a team with Sue Storm - wow she was something! I used to collect all the magazines with her.........Oh right, we were talking about Banner"
- The complete conversation between Cap and Bruce during the Battle of New York: "Hulk. Smash. Just like Ben Grimm used to do back in the day. Yeah, I missed out on a lot of stuff while I was in the ice. But I've been trying to catch up. Did you ever see him in person? Nah, you're probably too young. Anyhoo, go smash those aliens!