fifteen years (or so) and they have a new strangle-hold, no? (at least in their minds; i think things innovate too fast these days for this style to effectively work - it was a great 1980s move for them, obviously.)
but - in five years when the deal expires and microsoft has locked everything in as much as possible - what is the cost to switch out of it? i will bet that microsoft will draw up a new contract - paid this time - which will be less than the cost of full migration. then, in another five years, when - presumably - everyone is really locked in like they are with office - what will that contract cost?