DS-on basically isn't even a format rotation at all, it may as well be DS-on now bar literally a FEW cards.
OK let’s test that statement. In the latest US regionals we had the following decks finish top 4.
1. Flygon variants – This includes Flygon ex d + Eevolutions, R-Gon, and Flygon ex d + Metagross (dx & pk). R-Gon isn’t really affected but both of the other major variants could take a hit from a DS on format, though clearly the Metagross build is more affected.
2. Absolutions – Probably ok in a DS on format though loosing Espeon will hurt.
3. Metanite – It’s still not clear to me how much Metanite is going to be hurt by loosing Latios* / Ray* and Lugia ex. Even in a DS on format Metanite looses some power.
4. Delta – Unaffected.
5. Destiny – Largely unaffected.
6. Steelix variants – this includes MSN, Metalix, etc. Gone in the rotation.
7. Banette variants – Loosing Houndoom will be a major blow. I’m curious to see if other combos develop though. Still the standard Bandoom as well as some variants will be gone in a DS on format.
8. Shiftry variants – Largely unaffected.
9. Polistall – Killed by DP. RIP.
10. Flairados – Gone in a DS on format.
11. Meganium – Gone in a DS on format.
12. Raibok – Unaffected.
13. Mercury – TBD. Will loose Scizor ex & Steelix ex as options but could be really good in a DS on format.
14 Sallygross – Gone is a DS on format.
So out of 14 T-4 finishers this year 5 decks will vanish altogether, 5 or 6 will be unaffected, and the rest face some degree of change of strength either for the better or for the worse.
Granted I don't have the most experience but that seems like a pretty significant rotation to me.