Sometimes new stuff is an improvement (like the 375 Ruger over the H&H) sometimes they do something a bit different (300WSM goes in a short action) Sometimes they just "legitimize" and old wildcat. Sometimes they just make no sense whatsoever (the WSSM's come to mind)
And sometimes it's just a new option for those that may not have the cartridge that it is replacing- FOr instance if you don't have a 264 Winchester but maybe wanted something similar, the 26 Nosler may be an option for you. Like when the 300WSM came out, my opinion was that if you have a 300WM that you liked, no need to get a WSM unless you want one.
Since we haven't seen a mainstream 6.5 "magnum" size cartridge introduced in some time, it's pretty interesting, I think. Of course, there are those that just hate anything new.
I like them all- I use cartridges that were introduced 140 years ago, newish ones like the New King and everything in between.
With the current long range shooting trend, I think the 26 Nosler might be a good seller, but it's hard to say. Many new cartridge introductions in the last decade have not been great sellers, but it's usually easy to spot who the winners will be- they have to have a combination of appeal in caliber, bring something new to the table in regards to case design, and not be redundant to what is already out there - like how the SAUM's were. They were too late to the table after the WSMs got lot's of interest. Remington really dropped the ball on that. Lots of losers but winners like the 300 and 270 WSM and the 375 Ruger are doing really well.
Actually, when you think of it the .26 Nosler has a really good chance due to it's caliber- The 300WSM was a winner because it was a new .308 caliber, while the 270 WSM and 375 Ruger did well as they are the first improvements in older calibers for some time. 6.5's are gaining in popularity and it's a new 6.5 so it may have that rare combo of caliber and case improvement that people seem to be interested in.