Nah, they won't cancel the 3.75" line. Just call it an educated guess based on a lifetime of collecting and playing with these things. The 3.75" figures are Hasbro's bread and butter when it comes to the Star Wars license....everything else plays second or third fiddle.
Yeah, given that Hasbro currently makes their Marvel figures in different scales, a new 6-inch Star Wars line is likely to be an additional line rather than a replacement for the 3.75-inch line.
Besides, you know
a 6-inch line of figures will probably cost $15.99 to $17.99 each (at the very least).