I wouldn't consider "20 percent of customers were responsible for more than half the money raked in" as having "whales". At that point, you can usually still afford to lose one of your customers and survive. I've worked at companies where 1 out of their 15-20 clients is responsible for 25+% of revenue...THAT is having a whale.

If you do have a whale, then you've essentially just become a custom software/service firm for that "whale". From my experience working at companies that had a "whale", the business starts prioritizing the requests of that one "whale" over what's best for the product itself.