You are assuming the taxation laws stay as is - the government relies heavily on taxing the motorist, it needs the cash to keep the country on an even keel (although not entering into an argument about labour vs tories here), if everyone switches to EV where does all that lost revenue come from? You can only tax ICE cars so much, and if the numbers dwindle to the point all EV enthusiasts are predicting there will be a multi billion pound blackhole in the UKs finances that needs to be filled, which means switching the taxation to EV or taxing the electorate in other areas.
Take banking for example, was it 8-10 years ago a big campaign to stop them charging to use cash points and lots of people rejoicing at the money they saved. Now, 10 years on, the free current account I had 10 years ago with breakdown cover etc now costs me something like £10 a month. You will always be charged, just not always through the same channels.