Anything you can think of that's made of plastics was, prior to the mid 20th century, made of metal, ceramics, glass, fabric, or wood.
You'll realise these materials are usually much less suited to the task. In aggregate, the polymer industry was a huge benefit for humanity.
To take just one random example, I think electical wiring was insulated with fabrics in the early 20th century, which must have been expensive to make, and perhaps no too durable.
But really, plastics improved the manufacturability and performance of literally everything.
Lots of medical equipment is plastic. Most outdoor clothes are plastic. Many parts in a car are plastic. Suitcases, bags, etc are plastic. Shoes. Bookbinders, covers, cases, just look around the place you are right now and you will notice more plastic the longer you look.
I see the following advantages:
- Onetime use disposable
- lightweight
- durable
- manufacture is easier and cheaper
- often the better solution than the previous one
Based on the comments, if we accepted life is harder, life is shorter, and an individual's geographical world is smaller, plastic wouldn't seem so indispensable. I am not ready to accept either except for maybe the last one myself.
Ask any nurse or doctor. There is somuch single use plastic used in the medical field that I don't think could be practically replaced with glass, paper, hemp, or something green.
Of course plastic is also lightweight and relatively durable. This makes our cars and airplanes more fuel efficient and environmentally friendly, among millions of other uses.