Well, I absolutely agree that there is no way a passenger or ground based computer hacker should be able to access any aircraft operational or flight systems, and that dependence on firewalls is utterly insufficient. Any critical systems should be isolated from any method of corruption. I will not fly any aeroplane which has its FCS, FADEC, nav, fuel, hydraulic, electrical or pressurisation systems connected to any kind of hard wire or wireless network that can be accessed by anyone other than a pilot, or an engineer when on the ground, and I don't think many other pilots would fly it either. I can't see many airlines wanting it either, if it was a real possibility, and the airline insurers wouldn't cover the airlines if they did buy it with such a flaw, so it'd never fly in service.
However, while the report states that the 787 architecture differs from current designs, it doesn't give any details of how or why critical systems will be hackable. It merely states that as a new, complex system, it could potentially be hackable. This is something that will require rigourous investigation and testing, and if it is found that the syatems are at risk of hacking, then the architecture must be changed. As i said, I think IALPA's stance is one of wanting reassurance of the unknown, while Airbus may be stirring up trouble for commercial reasons - both manufacturers have a long history of corporate espionage, under-table deals, mud slinging and so on (just look at the USAF tanker contract for an example).
On the plus side, there are many novel design features of the 787 which I hope will eventually become standard, like electric motors in the wheels to allow "push back" without tugs and taxi with only the APU running, electric compressors to replace engine bleed air for the cockpit and cabin pressurisation and air con, just for starters. It looks like a great aircraft, assuming it doesn't have any critical "bugs" like the one under suspicion in this thread.