Chargify isn't blaming their customers for incorrect assumptions, but their customers are feeling the effects of their incorrect assumptions.
With the way things are now at Chargify it costs more to maintain/support a current customer than that customer generates in revenue. You can afford to grandfather old accounts if you can still at least break even on those accounts. If you can't then you have to do something like what Chargify did or your business will cease to exist. The key is to do all that you can to not put yourself in the position that your support costs are so high that you can't be profitable on existing customers.
I blogged more about it here:
http://www.bluemangolearning.com/blog/2010/10/building-scala...