Except in the case of a natural monopoly (which doesn't apply to software) you can always switch to something else. Which means the seller needs to remain better than the alternatives.
You underestimate inertia, and the fact that the industry has trained people that they must pay continually. On top of that, the common practice is to make it sufficiently hard to get your data out of your system and into the competition's - even when there's reasonably priced competition it does a great job at adding to inertia.