As electric cars become more popular, expect taxes on them to go up as well. Germany recently scrapped fuel tax breaks for biofuel. In the long run, it's unlikely for tax breaks to be maintained on popular items of consumption.
Rational tax policy would levy taxes in proportion to the externalities produced by any given activity. Cars which emit air pollution generally produce more external costs than those which don't, so it would be reasonable to expect taxes on them to remain higher forever.