Even if EVs are powered by fossil fuels. A powerplan producing electricity and transmitting it to your house is orders of magnitude more efficient than generating energy from gasoline in a motorvehicle using an internal combustion engine.
Also we can curb emissions at the power plant and track them dramatically easier.
And more EVs does not mean more power plants turning on per se. Most EVs are charged at night when power usage drops dramatically (and when you get cheaper rates on your electricity).
The real answer here is using more green house gas free power options like nuclear and solar and wind. I doubt we'll see much nuclear in the near future because it is so cheap to create solar and wind farms at this point and there's dramatically less risk despite it being dramatically less productive.