That was the rule of thumb I heard quoted in Business Studies, too. I think the key point was that a customer base tends to have a lot of inertia. To overcome that, you need at least one element of your competing offer to be dramatically better, and for quantifiable things like a price, an order of magnitude is significant enough to achieve that (and by implication a factor of only say 2x or 3x wouldn't necessarily be enough to dislodge an established brand).
Clearly there's a lot more to market dynamics than that in practice. On the one hand, we have supermarkets competing over prices for commodity household products by the penny/cent. On the other hand, at least here in the UK, we have industries like mobile phones, personal banking and household utilities that are all notorious for having poor customer service yet also all have surprisingly low numbers of customers switching brand.
I'm not sure where on that scale a typical consumer Internet service would fall, but in all cases it seems highly unlikely that a service with an inherent element of lock-in (even if you can move your data, it's going to take time and effort to do it) has anything to fear from an occasional screw-up that affects a few customers. It's obviously undesirable, and hopefully none of us thinks it's OK to dismiss anything that damages any paying customer out of hand, but it's not some sort of doomsday scenario that is going to bring down a whole company overnight.