You're effectively arguing that there is no point in picking up your litter at the park because there is a pacific garbage patch...and man the garbage can is all the way on the other side! It's like a full time job to throw away my litter! Me throwing away my litter is so tiny in the grand scheme of things, so it's not my fault if I just toss it on the ground.
We both agree major changes need to happen. But every time you leave your litter out, you're contributing to the problem as well and shoulder some responsibility. It's not like throwing out your litter precludes you from pushing for systemic changes. It's just something you can do, right now, to minimize your contribution to it.
And buying from places like I suggest is even more useful because it helps them thrive and spread. While I don't see how throwing away your litter really helps it spread.