There is 'should' and there is 'does' :-) When IBM acquired Blekko they were moving people over to the web based version of Lotus notes at 1,000 employees a month (give or take) and with 450K employees that was a 30+ year timeline (it got faster as they worked out transition kinks but still) the point is that at certain scales these things are a lot more difficult for enterprises than you might expect.

The biggest challenge (as I saw it) was making sure that everyone could email everyone else all through the transition because if email stops, the company stops in that portion.

All of that to say that even though Airbus is one less than a third that many employees (133K vs 450k) its organization complexity is up there. And keeping complex organizations functional and efficient as possible during a large transition is often addressed by keeping the rate of change manageable (aka slower than you would expect).