Mega Man's evolution was called "X"
As well as "DASH", "Zero", "ZX", "Ryuusei" and "EXE", as well as the various spin-offs they inspired (Hello, Battle N Chase, Soccer, Arcade "Power" games, Network Transmission, and etc.). All of which could be described as different approaches that tried to change up the Rockman formula, to different degrees and just as many levels of success.
So, I merely say give credit where it's due. Capcom and their affiliates have definitely tried to do different things with the Rockman series. And even there, they couldn't please everyone, in spite of the different things that each respective series/game tried to offer. After all, ZX could be described as a game that tried to definitely "evolve" the Rockman series, and look what happened there, sales- and critically-wise.
I'll openly admit that while I would definitely welcome almost any new twist(s) that Capcom could offer for the series, going more conservative is likely to be Capcom's line of reasoning for going for the nostalgia. By and large, it works. And in Japan, especially, developers on that side have had a rude awakening that basically boils down to how they can't rely on the more "fair-weather" non-fans and kids to get sales, and the attempt to market to that demographic has only done more to turn off established fans. So, if you're a company in the interest of making money, what should you do? Continue to try and vie for a demographic that couldn't give a rat's ass, no matter what you try to do to appeal to them, or go more for the "true blue" who are practically guaranteed sales as long you don't [acid burst] them off too much? The answer should be clear, especially in this economy.
Which wasn't much of an evolution in the first place.
Well, which is it? You just said...
I'd like to see a good and steady evolution
And X was definitely an evolution of the classic Rockman formula. By virtue of giving the main character new abilities, it incited a whole new means for level designers and play testers to work towards the goal of balancing a game play experience. Isn't that EXACTLY what you wanted?