If at some point in the far flung future, things like dirt bikes or even track monster race cars owned by folk turn out to be the biggest environmental problem, fine. Then we'll deal with it. But from my perspective we have an entire ocean of fish much bigger and far more worth frying first than them.
Now despite still enjoying riding a motorbike I am doing more recreationnal cycling and only transportation focused motorbike riding[1]. I just embraced moving a but slower. Turns out that with a simple bicycle you can get way enough thrill in the downhills, and with much less risks. As an ex elite road cycling racer, I am no stranger to catching and overtaking cars and motorbikes descending mountain passes.
[1] I basically only ride my motorbike if I have my partner with me and I am going to a specific place we wouldn't be able to make it and back home within a time constraint.