'Bikes cause it too' is technically true in the way that a dripping tap and a burst dam both cause flooding. The effect of this framing (intentional or not) is to suggest we shouldn't prioritise the thing that causes 99.99% of the problem until we've solved the thing that causes 0.01%. That's not a serious position, you're just protecting your comfort.

Have you ever seen the damage a dripping tap can do? I have seen houses completely wrecked by it.

Stop protecting your argument by trying to say something small doesnt matter. It all matters, regardless of size.

If the argument is that microplastics are harmful to health, surely we should be looking at all sources of them instead of just the biggest?

My argument is 'Lets try to make all vehicle tyres and brakes from something that doesnt pollute the earth and harm animal life on it.'

Yours seems to be 'Lets ignore the plastic pollution from other transport because cars cause the most, so thats all we need to worry about.'