Rather than forking and maintaining this old theme (if you must stay on it after the 1 year) for however long perhaps it might be worth it ( especially if you have 10 sites for example) to create a Canvas child theme for Storefront instead?

The appearance could be replicated from a client point of view. If I was in this position, this work would be absorbed as a cost of doing business if the choice to use Canvas was that of the agency and not the client.

However, there are probably those that aren't developers who maybe oversold their abilities? Since Woo has given people a fair bit of notice - Now might be the time to learn to do for yourself, hire someone at your own cost as a lesson or come clean to the client. If none these options sound appealing then yes, the clients will probably walk and they probably should.

Wouldn't this help more people move forward than being stuck in the past?