Not sure how it devalues Bitcoin. It might, but it's not immediately clear how. What it does is drive up the costs of running a node. Which is why those running nodes are incentivized to mitigate this attack in any way they can. That said, it's also valid that having to re-download transactions that were already filtered out is an impact. And these get in thanks to nodes which act against their own self interest. Sort of like cancer cells. The only way I suppose I could see a case for Bitcoin's market value being impacted though is that being costlier to use is a deterrent at the margins. But I think you'd be hard pressed to find anyone who's actually buying any less due to the prospect of higher fees in the future. Perhaps there's an angle I'm not seeing though.