We'll see what happens as Hasbro definitely is entertaining this new scale (which with Guardians didn't seem to move, but not sure) but I think if Hasbro were intent on abandoning 3.75 for SW, they would have done it. They seem to be investing further into Saga Legends / Mission Series into 2015 and I can't picture a scenario where they abandon that all of a sudden come September. Because it stops selling? 3.75 hasn't 'sold' properly since 2012 at least, and they still make it in spite of the market. Mission Series where I'm at least trumps everything else, and I can see this persisting in lieu of single SL figures; it's a better value for the consumer and as we've already seen it creates army building and repetitive buying potential, as well as cover for figures that may appear to be traditionally unappealing (girl figures, though I've never bought this concept myself).
Avengers/Marvel seems to have a particular issue at 3.75 that SW does not. Those figures simply don't sell. It's the same figure of the Hulk or Iron Man that it's been since say Iron Man 2 or early Marvel Universe and nobody is buying them anymore. SW is a universe which generates massive amount of unique characters and diversity and lends itself to more variety in the toy aisle. I was sad to see Guardians didn't get a 3.75 line as that movie was made for it but oh well. Maybe 10 years from now Funko will make one.