There would've been Klingons with ridges around during the 5 year mission, we just never saw them onscreen. That's how I rationalise it anyway. I think what made the make-up change acceptable in The Motion Picture, is there was continuity elsewhere. Their starship designs remained the same basic configuration as shown the last time they were seen in "Elaan of Troyus" etc. What Discovery does with them is more radically different and almost organic looking, rather than technological... instead of trying to produce a backward intermediate stage between the D-7 in TOS and say, the Raptors, or those baby Bird of Preys, or any industrial looking ship with added pipework along the neck to the main body from ENT. Klingon tech now seems to look like they're in the belly of some space dwelling lifeform. Everything is spikey and skeletal. Intricately carved out of something that perhaps used to be alive, rather than built. Rather than plundered from other races, as I've always suspected of the Klingons in the Prime Universe. The Augment virus wouldn't have afflicted the whole Empire. Indeed I think the Enterprise episodes only deal with Koovart colony... or however it's spelt. Despite this, I do like the idea of Voq/Tyler leading a resistance on the Klingon Homeworld, eventually giving rise to a controlling house full of smooth headed Klingons on the High Council. But on Q'onoS, I still think there should be a race more resembling those throughout TNG/DS9 and already retconned by ENT in "Broken Bow". In every interview I've read about the new redesign - and decision to lose the huge mane of hair, so we can imagine the pointy heads and extended ridges were always there underneath - has stated different Klingon houses have different looks. So, it would be nice to show that variety with one or two bearded Klingons... even if those without ridges is out of the question. Well, yes, Star Trek Discovery is being a bit selective about which bits of continuity fit in. Lines of dialogue are fine, if they are in passing and don't really have any bearing on the story.