Definitely fair points, and ones worth considering. I was speaking purely from my point of view and anticipated needs, which will likely vary for other people.
For example, while we don't currently have kids, I'm thinking that will change in the next few years, so second row room, seat adjustability, ease of access, and ability to install car seats will be important (for us); these are all factors that the reviewers in the above video called out as being worse in the LX600 vs the LX570.
I found it interesting that the reviewers suggested the LX600 has significantly better fuel economy than the LX570. They said they usually see around 13MPG on the LX570, and around 17MPG on the LX600. Interestingly enough, I'm right around 17MPG on my LX570 (mostly in city driving with tons of hills), and have seen as much as 19MPG on flats/highways. In the real world the difference between the two platforms will likely not be huge, and its benefits will be lessened by the smaller gas tank in the LX600. Now if the LX600 came with a larger tank that would allow for more driving between fuel ups, that would have been a big win IMO.
Improved ride and noise levels are subjective, but I'll agree that newer vehicles tend to be better in both areas than previous generations.
My office-chair impression is that the LX600 would be a better vehicle for those with either no kids or older kids, and who stay primarily on pavement, and the LX570 would be better for those that spend more time off pavement, and have younger families. I'd definitely be interested in taking the LX600 for a test drive, though.
I'm curious - have you recently driven a late-year LX570 reasonably back-to-back with the LX600? I only ask because that would put much more emphasis on the improvements in driving experience, interior quality and noise levels. I've got a 2014 LX570, and I suspect (hope?) that I would similarly notice a large improvement in the LX600, but I'm not sure how much of a difference there would be between a near-new, low-mileage LX570 and LX600.