This is something you absolutely must take on the chin, and whilst I understand your point - it is not always possible to provide visible updates to everyone and nor will everyone read them. There are updates on Michael John's site, albeit a bit old, but I don't believe he's someone who would just walk without trying to address things.
If you managed to get your money's worth from the app, then that's at least one good thing. I would definitely recommend purchasing Adriano's app.
Yet anything in the real world can and will happen, it's not always as simple as leaving updates and as you mention. Devs won't simply give their resources away for free; it takes a dev to pick them up and to pay a fee for them. In most cases, I think it's more common to see a new resource created from scratch.
Keep in mind that some Dev's have died, others have left in bad ways, but Michael is between this as far as I can tell. Whatever has his focus must be very important to him.
I get what you are saying, but it's not something to be expected.
It's a gamble on our part; you pay for the resource as it stands and Michael is one of those devs who add features to improve his work without additional fees. Whilst an explanation to say "I am unable to provide updates for the foreseeable future." would be nice.
There are no guaranteed expectations.
I don't think we as clients have that right either, at least not to demand it. As I say, I get the frustration, but I sway more towards concern than outrage.