I don't believe for a moment that we have reached "Peak Brum", but certainly there was probably too much capacity too soon on some routes, not only for the carriers but also for the terminal infrastructure, so a fall back in PAX numbers might prove a blessing in disguise.
NAX has retrenched from the UK / Med market generally, BHX isn't the only victim, and the Vueling situation appears somewhat similar.
There are plenty of possibilities for new destinations that would probably work with the right carrier and frequency, but through no fault of the airport, too much capacity has hit the traditional bucket and spade destinations, and this has now been (over) corrected, probably temporarily.