This would be treating a hypothesis as fact. That isn't scientific enough, it would be good to isolate a demographic where plastics are least used and a demographic where plastics are highly used with similar populations. Try to remove or invalidate external effects or properties and calculate the health of the individuals. Health not just alzhimers, because everyone varies differently. This would be almost an impossible task.
I'm sure there are more scientific ways to do a study.
No. I'm not saying this either. I'm saying with some evidence it's worth investing more money into causative experiments. This study is that "some" evidence.
That means double blind experiments. Your correlative studies don't offer enough evidence. Causation is our strongest scientific metric.
Human causative experiments are worth it as well in my mind with paid volunteers who are aware of the risk. But the legal barriers here are likely high.
I'm sure there are more scientific ways to do a study.