To be fair, they have been charging from day one, through other offerings. Although Trello is a free product, it's maintained by a VERY profitable bootstrapped company. They have a brand to protect. Killing a product used by half a million people seems like a terrible idea. It seems like they should integrate their paid products into Trello and cross-promote with paid services.
I'm usually right there with you on the "free vs paid" bandwagon. In this case, there does seem to be a clear disincentive to kill a product that would probably destroy the reputation of a profitable brand. I do think it's a dumb idea to count out ever charging for a product. If people are willing to pay, that should always be an option. Also, as a free service for business users they should definitely find ways to make users data completely portable. I'm much more comfortable using a free service if there's a data portability option.
UPDATE: There is already a data portability option.
"Portability
Your data is yours. If you ever want to stop using Trello, you should be able to download your data. All Fog Creek products feature data portability where applicable. We strongly believe in it. To export your data, simply open a card or click the board title, then Profile. click the 'Share, Print, and Export' button, and you'll find export options. Currently, we allow export to JSON which is a format that lends itself well to technical usages and cannot be loaded into programs like Microsoft Excel. It is, however, complete, and so does suffice as a backup."