This doesn't really apply because harm to a pedestrian during an impact isn't a linear scale.

There are sharp decreases in fatalities and permanent injuries, particularly to children who are often the ones hit in neighbourhood streets, below about 30 km/h so there's a strong incentive to have drivers travelling at speeds no higher than that to avoid child murder and maiming due to inattention.

Below those speeds, and given that people do often belatedly apply the brakes when they're driving recklessly there is a much weaker case for further reduction in speed limits. At least until car geometry changes again to make them even deadlier /shrug shrug

or so he claims, even if we assume he is lying about that

...what reason would we have to assume this, and why even bring it up if there isn't reason to?

I'm aware that the person is disabled, but they explicitly stated in their comment that "there’s currently no reason for someone to switch to public transport" and that "all this is going to do is annoy and upset people". That to me implied that they didn't think of cycling as an alternative to driving a car at all, which is understandable given that they probably can't cycle. So all I did was offer the perspective of someone that does cycle and has to ride on busy streets because there are no bike lanes in hope that they'd appreciate that perspective just as much as I'd appreciate someone giving me their perspective when I'm missing something due to my personal circumstances.

I agree that there need to be alternatives to cycling in place for those that can't or don't want to cycle. Its all about capacity planning, though. If you assume that 20% of your population will use bikes for short to medium distances you'll have to plan with much more frequent and larger busses or even consider building subway or train infrastructure to get all these people to their destination than if you assume that 70%-80% will use their bikes. So as a transportation planner you'll need to make a judgement call on how you want the future of transportation to look like in the area you're responsible for, but I don't think the choice to make biking the backbone of transportation is necessarily to the detriment of everyone else. To the contrary, the public transport might be less crowded if most people use their bikes, so that for example folks in wheelchairs or those with trouble walking have it easier to get on and off.

Given the state of public transport in Wales and other rural areas (hello from Cornwall!), it seems the only benefit from the reduced speed limit will be making it safer to walk or cycle short distances. Speaking purely from my own personal experience, I'll feel much safer crossing the road to get to the village shop if cars are going 20mph rather than 30mph. That said, it'd help more if 30mph speed limits were actually enforced. Plenty of people bomb through my village at 40+mph, and seem no more likely to obey a 20mph speed limit than the existing 30mph speed limit.

They're not just lowering the speed limit. From the FAQ:

Will the roll out involve money being spent on speed bumps?

There is no plan to include traffic calming (including speed bumps) as part of the change to speed limits. There are other ‘softer’ measures that might be introduced, such as using buffer speed limits, removing the centre line, narrowing the carriageway visually, using planting etc.

Ok then drive slower it's a speed limit not a speed target

Oops yeah, totally misread!