I would say £15 for a graphic equaliser is expensive. Most of the presets are pointless, which is common to all EQs.
The reviews for the original Boom were poor and deteriorating because of all the bugs. Instead of putting in effort to fix the problems and support their customers in a normal fashion I suspect the developer abandoned the original version to lose the bad reviews. This is pretty low, bordering on kitten murdering territory.
The way I look at it is that once a scumbag, always a scumbag. They have screwed their users over once so they are likely to do it again, wait until they release Boom 3 and abandon the current crop.
I don't mind developers charging for upgrades but they need to meet reasonable criteria. If I'm paying £15 for a very simple app then I expect very good support, Boom failed at this first hurdle. Don't leave your old users with a completely unusable mess. When you charge full price for an upgrade add some new features! Boom 2 added exactly zero new features, the only change was a flat graphical redesign to match Yosemite.