That and the fact that clinching does show up when Boxers or Muay Thai guys fight, and pummeling does show up when wrestlers compete. When was the last time you saw anything that looked like Chi Sau show up in a sparring match??? And I know! I know! The response is going to be "Chi Sau is not fighting. Chi Sau is for developing attributes used in fighting. Why would you expect fighting to look like Chi Sau?" My answer......people have tried to make a direct comparison between Chi Sau and things done in other fighting systems. However, as NI suggests, the platform used for clinch work is the clinch! And the clinch is something that happens in Boxing, Kickboxing, and MMA. The platform used for pummeling is the "tie up" position, which is something that happens in grappling. The rolling Poon Sau platform for Chi Sau is very artificial and doesn't show up in sparring. So it is not really directly comparable to clinch-work or pummeling as suggested.
However, it can be trained more realistically. DanT's school seems to do so. I know Rick Spain's guys in Australia do so. And using the Poon Sau rolling platform as a transition to these close-in grappling kinds of applications doesn't take a "high level" training in playing the Chi Sau game that so many work on. It just serves as a good "jumping off point" when contact is made with the opponent.
Another factor to consider, you can take it or leave it, is that JKD schools have never emphasized Chi Sau to the extent that Wing Chun schools do, and many of the JKD schools today don't seem to do much of it at all. Some have dropped it completely. Why is that? Because using a practical mindset they came to the conclusion that it isn't a high yield exercise to spend time on when it comes to being able to fight effectively. And every JKD school I have seen seems to spend far more time on actually sparring than on doing Chi Sau. Take that for what it's worth. Just another data point in the discussion.
The same can really be said of clinch work, as not all clinch work is created equal. The objective of the plum is not to tie up and exchange, it's to strike dominating the center and interior position, much like chi sau. So I would posit that it's directly comparable. There are multitudes of plum techs that don't require you to grab the guy and be tied up in a "clinch" to execute, so that is actually not the correct platform. What you and NI are referring to as "clinch" at least in relation to Muay Thai, is a by-product of not successfully controlling the center/interior position or having your attempt nullified. Plenty of examples of this in Seanchai, Pajunsuk, Samart, etc etc. And chi sau as I was taught is quite the same in that striking with control of center is a main objective, establishing the bridge. One problem that both of these mechanisms can develop if not trained properly is "chasing hands" or "chasing plum" where a person forgets why he's there in the first place. And perhaps that's what you're alluding to. But as a training mechanism, I position that they are comparable and valuable. To say that they are not is questioning a fundamental understanding that is at the basis of both of these training methods.
Regarding the data point of JKD's reduction of chi sau, its much more quantified than that. According to several who knew and trained with Lee, he personally continued training chi sau even after closing his schools, teaching privately, and up to his death so that closes the case on the speculation. But since you referenced it observing that I practice, I'll extend the point. JKD at the heart of the reduction of chi sau, lies in refinement of the strike as an establishment of the bridge. So the attributes of chi sau are still present as a by-product when this isn't achieved, at least with places that do their homework.