Suppose I get upset about all the fossil fuel which is being burned and decide to do something about it. I buy up fossil fuel and instead of burning it, I sequester it underground in such a way that it can't be used by drilling companies. This seems clearly good for the environment, no?
"Wasting" plastic does exactly this. If you buy ordinary non-biodegradable plastic, and dispose of it in the garbage so it goes to the landfill, you are effectively buying up fossil fuel which could have been burned, paying someone to transform it into solid form so it can't easily be used as fuel, and sequestering it underground.
Of course it is not a complete solution to global warming. But it is something we should encourage people to do.
So while this won't solve it on its own, neither will anything else. We need to start doing what we can and stop worrying about whether it will be enough in isolation.
That's like trying to plug a fist-sized leak with a toothpick.
It's not nothing, but it's not a good excuse to waste more single-use plastic (as you're implying here saying "Single-use plastic waste is 'good'")