Rather big difference in my book...
Xindi attacked Earth, killing 8 million people. In the grand scheme of things, this is literally nothing that would have a major impact 100-200 years later.
I think that there's room to rectify things. We know from TOS: "Errand of Mercy" that the Federation and Klingons were still negotiating to avoid war, or maybe it's a result of
this war we're seeing in Discovery (the two Koreas are still technically at war 60 years after their war, where South Korea ended up losing about 80% of its territory at one point, but if you look at South Korea today you'd never know how far the North actually occupied at one point). We could be looking at a similar situation; we could be looking at a temporal solution. Since the show is a serial and we have two episodes to go, we'll have to wait and see.
why was the USS Franklin (or Frank Lin) Starship Class in Star Trek Beyond?
It could also be a generic term to describe a vessel as a starship that was used in early Starfleet, but I agree having the Franklin listed as "Starship class" doesn't help keep things clear. It took something that we could easily explain as TOS not having ironed out all the details of the universe yet and kind of threw a wrench in it.
The point is, on-screen during the 3 year run of TOS, it was never once officially referred to as a Constitution Class, regardless of it being "retconned later"
If you really want to get technical, the
Enterprise was listed as "Starship class" during TOS. We never saw the dedication plaque of the other Constitution-class ships. We only saw the Defiant's in "In a Mirror, Darkly", where it was labeled as "Constitution-class". So maybe the Enterprise had some kind of special plaque? Or a misprint that no one bothered to change? I understand this is reaching but when you go for every single detail you kind of open yourself up to it.