The problem with Y2K was that computers would say the date was 1900. That might be a little annoying, but it would not interfere with much of anything, especially not autopilots or anything else critical. Maybe people's taxes and other records, but not mechanical-control systems.
Unless it makes the program commit a fatal error and be terminated.
A quick google search for "boeing Y2K" lead to this interesting article.
Boeing jets built before about 1980 are not affected because were not designed with any onboard computers, officials said. Boeing did not study non-standard equipment such as in-flight entertainment systems.
Boeing engineers said they initially feared the Y2K problem would affect virtually every computer system on the airplane.
``We assumed the systems would not function,'' said Jim Huffaker, lead engineer with Boeing's Y2K team.
In fact, engineers discovered that onboard computer systems rarely have time-sensitive functions beyond recording the date transmitted by a central computer.
Onboard navigation systems were the main exception because they are updated every 28 days with the latest information on airports and navigation markers around the world.
In one such system installed on many 747, 757 Md 767 models, a computer screen indicates ``Nav data out of date'' when year 2000 information is fed onto the airplane.
In another case affecting some older 727, 737 and 747 models, the problem is severe enough that airlines would ground the planes unless it were fixed, Huffaker said.
In bulletins sent out last year, Boeing specified software or hardware updates to resolve the problems. The repairs are not mandatory because airlines can choose to validate their data using a separate code number on the in-flight display.
Boeing encountered its first Y2K-related problem in 1993, when a computer program crashed trying to order a supply of titanium for delivery in 2000, said Willie Aikens, program manager for Y2K issues. Since then the company has gone through all internal computer systems and intends to be ready for the new year by July 31, he said.
While the had (reasonable) concerns, they found out that the worse problem would be a specific model (20 of which worldwide) unable to take off due to inability to complete preflight checks. I am curious as to WHY it was unable to complete preflight checks, and what would have happened had it already been in the air at that time. 20 planes worldwide aren't many, but still an issue.
Most flight systems were unaffected, or will continue to function fine while only displaying the incorrect date. But that does not diminish the potential threat posed by such a bug. a combination of luck and preemptive action does not mean everyone who applied their brain and predicted a POSSIBLE failures was off their rocker. (Although I never agreed to the idea that EVERYTHING will fail, that was stupid)