It's not that switching completely to a nationwide popular vote would be worse than what we have now. It's that partly switching to a nationwide popular vote, on a winner-take-all basis, is a broken solution that will make us little better off, and may have unforeseen consequences; but once it's in place, any better reform will have to overcome the "But we already fixed it once!" obstacle.
I don't think there's any way that proportional representation could be abused to get some nobody elected. They'd still have to get more electoral votes than any other candidate, and in a system where electoral votes were mandated to be allotted proportionately to the state's popular vote, that would still require that they win the popular vote. Frankly, if a write-in candidate DID manage to win under a proportional allotment system, it would mean that he was popular enough that he'd probably still have won if backed by one of the major parties.
I frankly can't say I consider it would be anything but good for the US to have a President beholden to neither of the major parties. I don't think it'll happen any time in the foreseeable future, but if it did, it would probably be the key to breaking the current two-party stranglehold on elections at the national level. (I don't know how closely you've looked at the system right now, but the Democrats and Republicans have basically managed to rig the system so that if you're not a Democrat or a Republican, you stand very little chance indeed.)