Yeah, I would say that through X4, the Mega Man X series had a reasonable difficulty level. Not easy, but not impossible. I'm sure that if I actually got to the final boss of X1 again I could beat it now.
For the original series, I'd say that that line was probably between Mega Man V (for GB, the final 8-bit Mega Man game) and Mega Man 7 on SNES. 7 is not impossible, as I have at least gotten through all eight main boss levels, if not the fortress, but it's harder, and it changes Mega Man's size, animations, and gameplay style fairly significantly. MM&B and MM8 extended on that style. I greatly prefer the original style... 8 particularly has an annoyingly slow Mega Man, but I don't really like the style used in 7 or MM&B either. It just doesn't work as well as either NES Mega Man or SNES Mega Man X.
And then 8 is even harder than 7 (though I have also gotten through the 8 main robot masters in 8, it wasn't all that fun really...), and MM&B really launched off the difficulty up to "way too high". X4... I played it years back, but I remember finding it fun, and beating it with Mega Man. I got pretty far with Zero too, but it was harder with him... but X4's difficulty was doable. X5 was too, until I got to the Black Demon... I'd found it challenging, but playable, up to that point, but I just found that boss completely impossible. I think I only managed to even get him past half health (when he changes attack style and stuff) once or twice, and that was with several E-Tanks (which you then have to incredibly tediously fill up after you mess up your one chance at killing him with them). Bah.
... and then X6 was basically "a whole game starting at that difficulty level", and Zero and Network Transmission seem to have gotten their cues from that game and MM&B. There is such a thing as "too hard for the game to still be fun"... I think Mega Man crossed it.
But you are right that SNES MMX hadn't yet.