If the world economy could eliminate your job without missing it, then you are no longer generating trash to clean up, and that job goes away too. The person who cleans up after someone genuinely productive therefore contributes more than someone who cleans up after someone useless.
This is, no doubt why we pretend with all the might of imagination that certain jobs are critical and vital to the economy. The idea that double-digit percentages of the whole world economy could actually just be wasted on useless navel-gazing nonsense is just too horrible to consider. It may be true, but if we believe it is true, that just means we're all wasting our lives on pointless bullshit. I'd rather lie to myself and be happy, regarding this particular point.
Our role as the elite is to elevate and recognize their worth, and not sink them to our moral level.
Also, when thinking about creating value, you need to think about scale. They change your trash and are limited by their physical ability and time. Let's say they saved you 4 minutes and changed 400 trashes, saving 399 other people 4 minutes. They've created ~24 hours of value.
If your app helps people save 30 seconds a day and did that for 100k people, then you created about 12 days more value.
At the very least the lady emptying the trash keeps the place hygienic, reducing the spread of diseases.