...which are being immediately shut down.

Depending on how you define "immediately". This is from the Atlassian FAQ:

  The end-of-life dates for each products are below:

  Stride: February 15th, 2019
  Hipchat Cloud: February 15th, 2019
  Hipchat Data Center (v3.0): June 22nd, 2019
  Hipchat Data Center (v3.1): September 26th, 2019
  Hipchat Server (v2.1): December 8th, 2018
  Hipchat Server(v2.2): May 30th, 2019 
  Hipchat Server (v2.4): June 30th, 2020

Their official post says they are working on a transition path:

https://slackhq.com/atlassian-and-slack-partnership

Yes, I should have been more clear.

They are immediately deprecated. It doesn't appear that Slack has any real interest in Hipchat/Stride tech (I know I don't).

Okay, so given that Atlassian has a clear and direct reason to develop a competitive chat application, presumably they'll embark on producing a new solution and marketing it to their customers as a competitor to slack? I'm not sure I buy it.