It was tough at the show, but the reason was that the vocals were partly mixed so upfront that your ears started ringing each time stretched a note. Another low point was him starting too late in songs. This happened more and more during the second half of the show and was something we weren't used to at that point. So it was quite distracting.

Getting back to the original discussion:
As of now (and having heard the Miami show) I wouldn't pay to see this band anymore either. And this has nothing to do with me not wanting to support them or being angry that Richie left. For me the weak link live is Jon. Simple as that. I enjoy the new album and the promo show in London was good, but he already had trouble with the new songs in Miami as soon as some of the older hits were mixed in.
Sure a live concert is never the same as listening to a SBD recording, but it can only cover the flaws to a certain amount. When I pay that amount of money I demand to be entertained and hear performances that take me away. It makes me sad but Jon can't deliver one of these aspects anymore. He's so occupied clinging himself to the mic trying to stay in tune that he doesn't move around at all. In 2013 I went to three shows: In Munich his voice was deteriorating midway through the show, yet he climbed the side parts of the stage and moved like crazy to keep the crowd entertained. That made up for the flaws of the band and his voice and led to an enjoyable night. In Stuttgart the crowd only was into the really big hits and Jon's struggles were obvious that night. In Cologne it was stunning, but even back then all the stars had to line up for a night like that: A setlist that was out of this world, a crowd in an excellent party mood and the giant echo of the stadium which helped him a lot.
Now you have to realize that his voice is about 50% worse than it was in May/June of 2013. I simply don't see a way how they could work an arena. To have a loud crowd drown your own vocals they need to be pumped, to get a crowd rising you need to fire up the hits, to find the hits you have to go back to the demanding older songs, to present them you'd have to able to sing them...and we're back at point one again. Furthermore he's exposed to the casual fans in bigger arenas who are used to him being able to work a crowd. 95% of the attendees only care about Jon. If he's glued to the mic and they don't even use pyrotechnics or other technical inventions to distract from it this will be coursed to fail I guess.
Maybe they'll all surprise us and pull something out of the hat that no one would've expected anymore, but after the last 4 years I simply can't see that coming.