Shed-on-a-Pole,
While I can see some merits in your arguments from a general point of view, I can't say the same about the details. For example, you say that to generalise about the behaviour of the market as a whole is hazardous, yet you are happy to claim that only a few will follow Ryanair to another airport, many will not!
Beyond nitpicking, with regards to your specific gripe of their reductions at Manchester, I actually think you might have fallen into the trap of believing some of the Ryanair 'public slagging' you love to hate. The continuation of the DUB-MAN route is all that needs to be said in response to your 'knee-jerk reaction' belief. I'm confident that if any of the other routes were performing in line with the DUB-MAN one, you'd still be able to book a ticket for it today. Announcing the switching of the routes to other low cost airports was largely Ryanair spin in relation to marginal routes - try booking a Ryanair flight from Liverpool, East Midlands or Leeds to Charleroi, Cagliari, Hahn, Marseille or Weeze right now and see how far you get!
Your 'corporate politics, not economics' comment is interesting. What percentage of global business do you genuinely believe is based on pure economics and not corporate politics? Very little I'd wager.
You say that there’s a finite amount of airports in Europe but there’s also a finite amount of Ryanair aircraft – comments such as ‘regular Ryanair customers such as myself are now cut off from convenient access to the wider Ryanair network (Dublin excepted) and as such represent a lost opportunity to the business’ would be 100% valid if Ryanair had an infinite supply of planes but given that they don’t, why shouldn’t they focus on the locations that currently give them the best returns (something generally not viewed as being detrimental to the interests of shareholders)? Don’t forget as things stand, the Ryanair as we know it has limited expansion opportunities available anyway given that they are fast approaching their planned total of 299 aircraft with no orders beyond 2013.