They should do three things with Damage Reduction, IMO:
1) With the exception of vampires and werewolves, they should remove DR entirely from all categories of monsters. If a monster is supposed to be extremely tough to kill, there's already a better mechanic for that - lots of hit points.
2) For unique, named monsters they should consider keeping DR (or, better, Vulnerability), but instead of having it keyed to one of five special materials (silver, cold iron, adamantine, magic, alignment) it should be tied to a unique named item/ritual/whatever. (Perhaps that demon lord is vulnerable to the sword of the paladin who beat him last time.)
3) Vampires and werewolves are special cases, because these are classic monsters with well-known and iconic vulnerabilities. (I think they're the only such iconic cases, but I may have forgotten some?) Here, I recommend taking a look at two recent films for inspiration:
I would recommend modelling this by eliminating the concept of 'silvered' weapons entirely, and instead having silver weapons that, if used, quickly become useless (like magic ammunition in 3e). Oh, and give the werewolves a very significant Vulnerability to such weapons, of course!
In game terms, that is of course easily modelled - an easy-to-use ritual that can be cast during a short rest. That way, you get the initial 'shock' encounter of vampires that can't be killed, followed by a rest, and then on with the fun vampire-killing antics!