I'm not familiar with Dynamic Media, but based on my reading of how it works, it implies that media assets can be loaded at any time after the browser has loaded the web page. In that case, there is no guaranteed way for you to know when the last media asset has loaded.
Even if you were able to detect when the last media asset is loaded, it could happen long after the browser has loaded the web page. In that case, you'd be delaying your pageview tag for a long time. That increases the risk that the user could navigate away from the web page before your pageview has been tracked, thus causing you to lose data about your users at that page.