You're correct, though I'm sure nobody with access to the actual calculation is copy pasting my solution into the code, so I didn't feel the need to go deeper than a broad-strokes overview.
For offhand doublestrike, having PTWF, Tempest capstone, Vistani, etc change the ratio might be an option to proportionately accommodate what they lost, i.e. Tempest capstone changes the offhand DS % to 0.6, VKF abilities give +0.03 per or whatever. Keeps their flavour and power relative to other TWF stable. Currently on almost any melee build you're going to be building towards 100% doublestrike, so slapping main hand DS on these abilities (which many have suggested) is a band-aid fix at best.
I think a lot of people complaining about the dervish change haven't considered that right now Tempest capstone dominates so heavily that there's very little reason to not go 20 ranger on TWF. Fighters, barbs, paladins, monks etc all see a huge playability increase with this change, and I think that is a great thing. Yes, they should make dervish worth taking, but looking at it even with this change it's still stronger than a lot of capstones are for melees. It's nowhere near as good as it was before, so I'm not saying it doesn't need some help, one just needs to take into account that TWF actually on the whole got a decent increase with this change, and adding back the same amount of power to dervish would just result in 20 ranger crowding out the other options again.