Regarding a new redesign, I would not believe it will be released in a few years, not even close, the new 3458A will be serviced and supported for at least 10 years, as if it were a new product, so maybe 10 years from now we’ll see a new 3458.
There are only a few options, 1) they either update and support the 3458A until eventually they don’t, and they get out of the business of making a 8.5 metrology DMM, 2) they make a successor for 3458A that is 8.5 but not metrology grade, kinda like a successor to 34470A, 3) they do a complete redesign and 4) a variant of 1) and 3) where they support the 3458A indefinitely (10-30 years), making incremental updates and localized redesigns over time, making the support scalable.
I see 1) or 2) as likely outcomes depending on business outcomes and net profit of Keysight and demand, 3) and 4) are related, and they would be driven by business assertiveness, strategic direction and top down directives.
Keep in mind, a complete redesign might very well be a layout and new BOM, but it will never mean not using, previous architecture and designs.
I for one hope for 3), and would not look forward to a brand new complete redesign, why? No transparency, no schematics, no component level support and repair and no parts, look at Fluke or look at the 34470, with the 3458A I can buy a new one and used one from ebay, transplant the parts, and go, that is great and unique, that can’t be said of Fluke, or any new test equipment or consumer electronics that comes out and sold today. If all the parts were off the shelf, purchasable from digikey in a quantity of 1, firmware was accessible and transferable, and not locked down and encrypted, that would be a step in the right direction.
The 3458A has seen a lot of updates, the digital board is gone and completely different from original, in time they can do the same to the remaining boards, but if they maintain the boards compatibility, that would be killer, no one has done or has that, except the 3458A itself, long term support and component level repair is the dream.