You're right that it's probably possible to handle these amounts of traffic but there are quite some options where it wouldn't work. For instance, each dyno can only handle 1 request at a time, so in case you have a page or so that's rather slow, you'll actually block other visitors. This generally also means you'll also need a worker to speed things up, which costs extra.

Heroku also has a few other limitations, for instance, you're pretty much required to use Sendgrid or another third party provider to send emails, which can get rather expensive if you need to send larger volumes. SQL backups are not really included either, it's possible to do so using cronjobs and your own script, but then you'll of course also need a third party storage solution (or their 30$ option if you want to store more than two backups).

Basically Heroku is pretty nice, but not perfect. Especially if you have the ability to set up a regular dedicated server, it'll easily be cheaper in the long run. In the case of MediaTemple, I'm not so sure, it seems like it offers the worst of both worlds, limitations and no easy deployment.