The bubble will burst and soon.
It's already happening. Major cable companies are already struggling with the rights costs of live sports due to cordcutting. As media moves away from the traditional conglomerates, the way live sports are handled will either change drastically or face reduction in revenues, simply because all the people that DON'T care about live sports are currently subsidizing those massive contracts. As those people filter away to an a la carte service model, the media companies will no longer have the user base to afford the current contracts. It'll be a bit before it starts to affect the PL, due to a global fanbase and thus more places to mine for rights money, but it will happen unless the leagues figure out a way to profit outside of the traditional forms of media.
That said, it's a good reason to build now, and 14 million a season should remain sustainable regardless of the bubble unless everything is broken forever and nothing matters anymore.