Uh-oh — ACR has gone missing

I have CC 2017.1.1 on my iMac with Yosemite 10.10.5 (14F2511).

Before I describe the missing ACR, I will answer what I think will be the obvious replies:

• I do not wish to upgrade my main working system three operating systems right now, and it is too late to have done it earlier
• I know I can do this on my laptop, which has CC 2018 and High Sierra successfully installed
• The new iMac Pro has been announced and is due out in December. I am keeping this system as it is until then (as hard as it is)
  

And now for the problem: I cannot open any any files that end in .nef, .crw, or .dng because ACR is gone.

I get this when I try anyway:

When I try to use the Camera Raw filter on a Smart Object in PS 2017.1.1, I get this error message

I have CC 2018 on my laptop, which I just had to upgrade to High Sierra, but I want to get Adobe ACR back on my iMac with the current operating system and with CC 2017. The problem started after 2018 was released, but not installed on this computer. It doesn't make sense that ACR is broken and I hope it's an easy fix.

Any ideas?