As we have seen with the Captain America films, Marvel have grown adept at preserving the elements of movies over a series of sequels. With such a wealth of source material to draw on from the comic books it is unlikely that their ideas will grow stale or repetitive with the next Guardians of the Galaxy film which is likely to be just as good as the original.
Guardians of the Galaxy 2 will definitely be as good as the original. There's still so much story to be told and so many other characters and plotlines. That seems like a franchise that can keep going and going especially because the actresses and actors in it are perfect for their roles.
Gardens of the Galaxy 2 has a great deal of untapped source material to draw from. One of the biggest reasons why Superhero franchises do so well, is that they can be creative and becasue they do not have regurigiate previous plots in order to create a good movie that people will like.
The first GotG was fantastic, so the filmmakers know what elements to the story and plot will work and what won't. However, we can't be sure yet, because the first sneak peak of GotG 2 didn't reveal much. It wouldn't be fair to criticize just yet. The con side says that "sequels are hardly ever good". This IS Marvel we're talking about, which has been killing it recently with the fantastic movies and sequels. I'm confident that this will be a great film.
No, it will not be as good as the original because sequels hardly ever are. They try to use the same plot too often or they try for too many special effects. The original movie brought in so many fans, but it seems unlikely that they will do an even better job.