Do we really need to pay $20/month just to have a chatroom? When there are free alternatives out there? When times are good, sure if you like, but when money gets tight, it's probably a contender for getting cut.
a) As stated in the post - people will look to cut FUTURE spending before discounting current spending. So the argument is that while getting new customers may be difficult in a downturn - the inertia of the fact they have to take an action to cancel - will slow down any attrition.
b) There's still a transaction cost - training, updating links, migrating data/history, etc with moving from SeviceA to ServiceB. Which will again push that migration down the list of cost-cutting priorities.