Western transition away from fossil fuel usage is still a reduction in net fossil fuel usage when compared to no shift at all. In other words fossil fuel usage might still be growing, but it is growing at a slower rate. If we accept that fossil fuel usage should be minmized then it very much matters.

Aside from that; this argument reduces to an exceptionally bad moral stance which is: Someone else is doing bad things, therefore that justifies me doing that bad thing. Or as you might phrase the counter-argument to a young child: just because someone else is littering doesn't mean that it is ok for you to litter.