What would be some possible effects, positive or negative, on life expectancy, if you genetically altered humans and animals to be able to digest and incorporate plastic into their physical makeup?

Erections lasting longer than 4 hours?

Already happening – people have replacement knees, hips, and various other plastic body parts that lengthen their lives.

And at least some portions of artificial hearts are plastic variants.

Now, if you’re asking about somehow reworking metabolism and the digestive system to be able digest and convert plastic – far too many unknowns. I doubt it could ever be done, chemically. Once you take the leap and buy into that possibility, you can imagine anything being possible, because it’s all science fiction at that point.

Cleaner oceans.