Well that's the thing, common players currently play at OD7 or OD8, there are almost no OD9 maps and lots of the more skilled players can play with DT or HR (both of which push the OD higher to 10 or beyond). With the implementation of decimal ratings then you can really get a variety of map settings now but it's still limited. OD13 would be reserved for top players since some people can hit -5 +5 (I commonly get -/+7 on good days with -/+10 on bad days). It's like how the star rating is set up now to where having 10 stars really shows that a map is really hard.
HR gives accuracy issues only because a person can't read it correctly, DT OD8 requires almost as much accuracy as OD10 and I've asked people to compare and they can do DT OD8 and they struggled at HR, it's the way the stacks are shown and how many notes are on screen that cause the struggle. Tighter timing windows wouldn't be impossible, just really difficult which is what they should be reserved for. Plus more maps would be able to hit those higher star ratings without creating impossible aim maps
I wish there was a way to have all the old maps read and shifted in settings so everything became compatible. Thinking about it, I do agree with full tablet about leaving the current numbers due to compatibility sake, but the current scores in place would be a problem to recalculate with higher OD with hard rock.
The end goal of this is just so there would be less mod usage and more focused skill playing. Too many players slap HR+HD / DT+HD on easy maps and drone out until they get good accuracy by luck/memorization, or on the other spectrum the top players get bored with how easy it is to hit HR+HD on maps (DT always gives a challenge due to high bpm). With maps that are on par with the difficulty of maps+mods then there would be less mods used and more reserved for highly skilled players, thus higher rewards are to be had.
OD11 ->OD8 [14ms no mod]
OD13 ->OD10 [6ms no mod]