For starters, if your shop closes their door... you are likely doing it because you ran out of money/time. Why would you go through the effort to package your work as open source code and help users figure it out?
Second, you can't just say "open source". You should specify a license, or at the very least say: "under a license approved by the Open Source Initiative".
Third, what exactly is open sourced? Are their proprietary services you use, did you license these for your users?
I suppose, if you want to make this promise, you could publish your entire stack on GitHub and put it under a license that its time-boxed: "If for any reason XXX stops operation, this source is released under the MIT; tell then, you may copy it only for archival purposes". But, in this case, what constitutes as "stopping operations"? What happens in an acquisition where the parent company is interested in some parts of the technology you built?
UPDATE: Edited to directly respond to _their_ effort to make this Guarantee to their users.