I'll be the first to tell you that I, personally, have plenty of criticisms with 9. But it was a nice change of pace as far as critical response goes, mainly because it was riding a nostalgia wave.
I don't really doubt that the established formula still works, although yeah, people are going to be reluctant to pay 50 bucks for it in this day and age (ESPECIALLY as long as Capcom is fixated on 8 bosses). The problem, as it went prior to 9, is that their attempts at it either failed to match up to their predecessors (Powered Up and its radically claustrophobic play style) or were completely bogged down with needless sidequests that barely gave you any worthwhile rewards anyway (ZX Advent is a serious offender here; near all of the chips are only available post-game and they don't do anything THAT special).
With 9, the NES style at least mandated that it was impossible for them to screw up the character-to-screen-size ratio, something that somebody really should have looked at more carefully with Powered Up. I'll never tell you that 9's level design was up to par with the actual NES games, though, because it wasn't. There's a difference between challenging and just blind-siding the player. And the over-reliance on the shop diminished the feel of legitimately growing stronger. And the nerve to charge for difficulty levels was Capcom's cheapness in full and obvious force. 10 was definitely an improvement, although the fact that it's the only game where I wish for a weapon to deliberately fire down a pit isn't necessarily a good thing, but other than that things were looking up at least quality-wise. Unfortunately, the audience didn't follow. Can't say I blame them, there were still a few kinks to work out. We as fans have a bit more patience, but if in two games of the same style you can't perfectly nail it, the general populace is going to very quickly lose their desire to see further sequels.