The rate of taxi replacement for old diesels is currently doubled, with the lifetime of a (Euro 3, 4, 5) diesel taxi reducing from 15 to 12 years over the years until 2022, so each year two years worth of old taxis will be too old and replaced by ZEVs or other low emission vehicles.

Yes, they're a big polluter, but they're also moving a LOT of people around the city, so that sort of cancels out. If anything the taxi drivers remain annoyed that this means they're going to be buying a (very expensive even after grants) new taxi sooner than expected.

The Mayor spent about £25M basically bribing those in younger taxis to upgrade to ZEV anyway, paying them over £10 000 each in some cases to swap from an old diesel (which would then either be scrapped or sold outside London since it can't hit criteria to be newly licensed as a London taxi now) to a PHEV.