You seem to be invested in the MCU as fan so I think that clouds your reasoning. The majority of guests are not fans, they just want to try something new. Once the buzz dies down it will be just another generic experience. In 10 years it will have the same crowd levels as RNR, which of course there are rumors going around for it being updated. Still don't think it will happen, look at Frozen, those wait times have shot back to reality pretty quickly. I'm sure TSL will be in the same boat too once GE opens if not before.
1). I'm not a hardcore Marvel fan, I just like every other movie and Guardians is one of them.
2). I think you'd be surprised to find out that a majority of Disney theme park guests ARE Marvel fans. Given the huge success and box office records, I wouldn't be surprised to find out that the majority of the American population in GENERAL are marvel fans. Hence, why Marvel Superhero Island has been one of Universal's most successful and popular theme park lands/attractions since it first opened, and why they won't back down on the rights.
3). In 10 years, the same crowd level as RNR would be pretty high up there, so we agree on SOMEthing. So yea, once the "buzz dies down," it'll still be holding an hour/hour and a half worth of wait time. Maybe more considering, again, Epcot has no thrill rides.
4). Frozen was not executed properly at all. It was merely an overlay of a pre-existing ride. Energy has been totally gutted, and Guardians is going to be one of the longest indoor roller coasters in the world. So again, you need to work on your comparisons.
5). Toy Story Land is yet another bad comparison, as the attractions in the land won't even come close to the level of a thrill ride as Guardians will. Guardians should be compared to Rockin', Space Mountain, Everest, and other more thrilling attractions. Specifically coaster style.
Whether Guardians fits thematically in Epcot or not will not hinder its success.