You're right, it really wasn't that strong to begin with. Everyone talks about Boeing's engineering culture but this all took place in the context of the 1990s

You're right, it really wasn't that strong to begin with. Everyone talks about Boeing's engineering culture but this all took place in the context of the 1990s after the DoD started to unwind its anti-Soviet doctrine and retooling the armed forces for other enemies. There was less money to go around in the defense industry and everyone was downsizing so the engineering culture at Boeing was at risk if not already undermined by layoffs. The 1990s also brought a global slump in aircraft sales which hit their commercial aviation divisions really hard and Boeing tried to age-out the most expensive workers that would have been the backbone of the culture. Lockheed had just merged with Martin Marietta to become Lockheed Martin and that goaded Boeing's leadership to blunder into a FOMO merger with McDonnell-Douglas. On top of that, the US government itself actually paid bonuses to the executives who got the LM deal done so the incentives were rotten from the start. I've also heard rumors that Boeing execs received pressure from the DoD to acquire McDonnell-Douglas to prevent it from going bankrupt so they might not have been entirely willing from the start. The latter had just lost some pretty large contracts to competitors and was at risk of going bust entirely which was an unacceptable vendor risk to the DoD. The DoD didn't have the money to prop it up directly so instead might have used a third party.