Many people were and continue to be happy with the Mk I, but I'd bet that those people
1) haven't tried a Mk II,
2) can't afford a Mk II,
3) aren't any kind of pixel peeper whatsoever
4) some combination of the above
I have tried the MkII,
I can afford several,
My smallest print is 24" X 16",
Personally I use sharpness as one of many lens characteristics of varying importance, out right sharpness is rarely a serious limit and few would argue the MkII has busier bokeh than the MkI. Lenses of this class become a matter of preference that goes vastly deeper than sharpness, the MkI focuses very fast, but with the latest bodies the MkII will focus faster, for sport and wildlife shooters that can be more important than slightly less smooth oof areas, for people who don't have the latest bodies the focus speed difference is not so apparent. For portrait and wedding orientated shooters busy and distracting backgrounds can break shots and take a lot of work in post to smooth effectively, besides, brides faces don't need the sharpness of the MkI, let alone more, I often have to dial in reverse clarity or do skin texture reduction to deal with the MkI sharpness levels.
I love new toys, I have tried the MkII several times but for me and my uses it is not an upgrade, just like the 1DX isn't an upgrade from my 1Ds MkIII, sure it does a lot of stuff better, but it doesn't do some stuff as well.