The kids killed Ben.
If he didn't become human for that one week, he'd have lived forever.
It's pretty likely that Ben could have found the tech to suppress his humanity if he did want to live forever.
Immortality is one thing, but getting "old"?
Quality of life dude.
Old people hurt all the time.
If the Thing hurt all the time because he was an old Thing?
Imagine the tonnes of opium poppies they'd have to stir into a thick custard for Ben to bathe in? And I'm not talking about about a spa pool, but a full sized Olympic bastard.
The Thing does not float.
Maybe he's buoyant in opium custard?