I'm sorry to hear this. In a subscription model world, Omni stand out in producing very high quality one time purchase software. I love OmniGraffle! Such a great product once you learn it.
Not really moving, more providing the option. Given that iOS and desktop are two separate versions, with new licenses required for upgrades...
(Maybe not the point, but maybe there wouldn't be layoffs if it was a subscription)
Unlikely. A few niche apps don't need development forever. Users have finite hours in a day, they don't have infinite appetite for more features. We are drowning in software.