If it isn't relevant, it begs the question why so many people, professionals in the field, still busy themselves with the topic.

It is relevant, exactly because professionals keep bringing it up, and more importantly,  because the issue on climate sensitivity has not decisively been settled (and also because scientists have staked part of their scientific career on it). Hansen's work in the late eighties is therefore essential for "proofing" ideas on climate sensitivity - it's an important test case for our ability in predicting the future.

As noted previously, Hansen considers his 1988 estimates of climate sensitivity now too large, and now puts the number for sensitivity lower. Whether he's wrong on that number now being too low, or whether it is still too high, the earth will find out as long as we don't switch to a society that is carbon neutral. In, say, 2028 it is likely we will be able to say something useful whether his 3.3 degrees estimate comes closer to describing reality.

The political problem of course is our political strategies for adaptation should be based on that sensitivity number.