That styrofoam you're worried about is the same stuff that's going to show up in a thousand years when somebody starts digging through a landfill, unchanged. Yes, a small fraction of an inch of the surface material may deteriorate in sunshine or ozone, but remember, that's also the stuff used to make coolers. It's an insulator, and waterproof (remember when it was common in disposable coffee cups?). As long as you haven't poured gasoline on the inner shell or dropped it onto a hard surface with a heavy weight inside (like your head), it's OK.
The outer shell only exists to hold the styrofoam together until it takes a "severe" impact. (And, of course, to bear the advertising and logos.) It protects the inner foam from things like tree branches, scuffing when you drag it on the sidewalk, dings from dropping keys on it, etc. If that outer shell is intact when you go for a ride, it's good to go.
Safety standards evolving? March 10 of next year will mark the 20 year anniversary of the CPSC standard that's still in effect. All that MIPS advertising? It's a gimmick based on a theory that has NOT been demonstrated in scientific experiments. (And no, you can't put on a white coat and hit helmets with random things and call that a scientific test.)
I use mine until one of the following occurs: (1) I'm wearing it in a wreck. (2) The pads fall apart and I can't keep it where it needs to be on my head. (3) Straps or buckles fail. And new the other year, (4) the outer shell detaches and is lost on a wonderfully fast descent.