Not at Shrewsbury, but I and others walked out of the final Fylde concert because it was too loud. Excessive loudness does reduce the quality of the music - if you notice the decibel level then you aren't paying attention to the music itself.

This does seem to be a final concert conceit: the same band (Eliza and the Ratcatchers) were excellent in their afternoon set. I honestly do not see the reason for it, and don't think the answers given above sensible.

If you walk out, you miss something you've probably been looking forward to all weekend, and that is why audiences don't, as a rule. But as long as those so responding are in a minority then organisers and arrogant sound engineers don't care when they do.