They are going to update sooner than later because it's not as secure as the rest of the Mac line. It's the only one without the T2 chip, besides the Mac Pro which will be replaced soon. I'm guessing after the Mac Pro is released a few months later there should be a new iMac with a T2 chip a slimmer bezel and no more fusion drive, or not.
I’m waiting for a redesigned iMac myself with T2. If we do assume the new Mac Pro and new Apple display gets announced and released later this year, the question is will Apple release the redesigned iMac later this year as well? I’m dreading waiting until 2020.