I'm not really sure what Flickr could even potentially have going for it anymore to 'become awesome again'.
I've never even heard of 500px. It certainly isn't at a point of making flickr a garbage dump.
Some of my friends and I have used pbase.com for almost a decade now and we prefer it over the alternatives such as 500px we've tried. There maybe folks who find it old or dated, but it works well for what we intend it for and, just like Flickr, there are loyal users who don't want or need to use the most cool or awesome site, because often they really aren't and are too much trouble than what we currently use.
Sounds like a potential (massive) advantage to me. Having your service be ground zero for export from your cam and contain all your content? That can be exploited to good effect.
This is one of the few areas where Flickr/Yahoo could still conceivably compete with Facebook; since FB doesn't have a "Facebook phone" yet, they'd both be in the same competitive position of needing to get camera and phone OEMs to buy in.
For many of us, Flickr's appeal is that it's (self-)curated: you've picked your best stuff to upload. Features like "explore" reward this; popular content is highlighted, while those who just upload every photo they take tend to not get any attention at all. They've effectively trained the userbase to share good content (insofar as the userbase is capable of producing, anyway).
It's the reason a lot of Flickr users, particularly the ones who produce good content, are interested in 500px: quality counts.
That dynamic would change quickly if everyone were encouraged to dump everything they take onto Flickr. It would devalue the service for many of us: nobody wants to see fifteen different perspectives of the same "moss on a rock", taken in rapid succession, but that's what your contact stream would end up filled with. You can only remove people as contacts for so long, before you decide it's no longer worth the trouble and jump ship.
Flickr isn't Photobucket or Picasa, and that's a good thing.
You're right that Flickr is more of a photodump than 500px...however, it is far more useful than just throwing the photos on Dropbox. Interface is everything, and being able to search by date, tags, keyword, and being able to group photos, or send them to groups, makes Flickr a wholly different service than one that is designed just to store files.
Services like Instagram and FB photos shows that users get value from places where they can indiscriminately upload photos. 500px is for pro photographers who want only their best photos on there.