What REALLY happened was not that Mashable failed. It's that the entire journalism sector failed. Mashable is just a good mascot and sacrificial lamb because it's actually one of the most well known ones.
I say Mashable is a "mascot" because they represent exactly what's wrong with "journalism" nowadays. Instead of producing what people should read, they have shifted their focus onto:
1. What people will click (Memes, controversial topics, content about other people's misfortune) 2. What advertisers will pay more for (Video)
It's understandable because it's been a while since "content" became commoditized. Now media sites no longer can differentiate themselves with content anymore and they can't figure out how to, so they just do what makes them money in the short term.
This article also inevitably mentions Buzzfeed, which I think is what Mashable would have been if it didn't have to go through all the trial and error. Buzzfeed probably learned a lot from Mashable's mistakes and decided to go all in on "I don't care about quality, I'll give you what you want, just give us ad money".
But those who've been following the media landscape also know that that model's not been working out either (It's a good thing that it's not).
So my take is, they didn't flame out because of all these little reasons. They were just the symptoms. The real reason was because they have lost their soul. It's not only Mashable that flamed out, it's that the entire media industry that is flaming out, and OP's own publication will probably flame out too, if they keep writing crap like this (I personally think all publications that monetize off of someone else's misfortunes deserve to taste their own drug and die off too)
Oh, by the way, this is nothing new, and is definitely not just because of the Internet. If you read up on the history of the media itself, you'll realize this type of pendulum swing between "quality content" vs. "shallow, give people what they want in exchange for revenue" approaches have existed several times already.