> If it's so PRO then why only a maximum of 16gb RAM?
They could have done, but it would have been a fatter, hotter and more power consuming machine, as it seems Apple might be waiting on Intel CPUs to support Low Power DDR4, which they currently don't. A 'Mac Flightcase' might be useful for some, but then if you have access to power on site and are already committed to lugging flight cases around, you might as well start off with desktop-class hardware.
Indeed, it seems that anyone wanting more choice in their kit would do well to focus their ire on Intel rather than just Apple - for years, Intel have been concentrating in low power rather high performance. Expect greater RAM support in 2018.
Editing 4K video is certainly doable on 16GB ram, especially if your SSDs are fast enough.