#2. It was a legitimate botch and the ending to Extreme Rules was called on the fly
Whilst it's unlikely, there's always the possibility that this was a legitimate botch and Finn Balor wasn't supposed to lose the match this way at Extreme Rules.
The Demon was about to deliver the Coup de Grace, and it's clear that he was unsettled on the top rope for a few seconds. Balor is one of the lightest stars in the company and it's highly unlikely that he'd break the rope with his weight alone.
That being said, when the ring was set up and checked ahead of the main event match, there could've been an oversight on the ring crew's behalf which resulted in the rope breaking.
The end of the match did look planned, but if Reigns knew that he was winning the match and Balor was legitimately injured from the fall, then the two men would've been able to call that ending on the fly.
With everything going on in WWE with all of the botches on last night's show, it's hard to completely rule out that this, perhaps, wasn't supposed to happen.