Actually, most professionals in the photographic industry prefer to use card readers rather than memory card slots in either a printer or computer. And they don't use memory cards for storing their images; they shoot the images and then dump the files into the computer, edit the images that they choose and safely store them on external drives and in the cloud. Memory cards go back into the camera for the next shooting session.
Sure, people who take snapshots with their cameras will probably be happy to have an SD card slot in their new MBP, but not a lot of them are likely to be buying the high-end MBPs anyway. Come next year when the next round of MBAs and the baseline MBPs are released, that's when there will be a larger audience for the SD card slot. Aside from that, the photographic industry is now moving more and more into CF Express Type A and/or Type B Cards, which are significantly faster than SD cards. High-end digital mirrorless cameras are now using CF Express Type A or Type B cards.
I don't think anyone suggested storing images on SD cards long term...just using them to get the media onto a computer, NAS, external disk or cloud storage (or all of the above!).
I too carry around an external UHS-II reader because a lot of laptops only have UHS-I or a poor implementation of UHS-II. I'm a bit disappointed that the MBPs are limited to 250MB/s, but it's still pretty good and close to the theoretical maximum of the cards in any case.
The built-in SD slot may end up being a back-up for professionals. Personally, I would always carry an external adapter as well in case something fails, but the built-in reader would be the easiest option in the field (less risk of losing the dongle).
I don't understand your point about the next MBAs and MBP13s (if they exist) - why would that increase the audience for SD slots? I doubt the MBA or entry-level MBP will have one.
You are correct that high-end cameras are moving to CFExpressA & B, but that still leaves the majority of cameras using SD! It will take a long time for low to mid range cameras have these (and that's where the numbers are), but I look forward to it eventually.
I know I sound like an "SD defender" of sorts, but I'm actually impartial. I have a camera with CFast 2.0 and external SSD, but also have 5 cameras with SD card slots, and these are a very good and mostly economical option for nearly all of my usage.
As with all things, you have to look at the requirement for them. Do I need to read/write to my media at 1000-2000MB/s ? How fast can the camera actually write? (hint: it may be a lot slower than the 1-2TB/s
). A $50,000 RED Weapon 8K camera has a maximum data rate of about 300MB/s. Most ARRI cameras are similar with the top-end Alexa 65 in open-gate RAW topping out at about 750MB/s. ProRes 4444 or RAW tops out around 500MB/s
However, you can still shoot 4K 4:2:2 H.264 or H.265 videos at 60fps at less than 50MB/s that are good enough for broadcast or cinematic release, and that is easily done on a $40 SD card.
I did recently learn that CFExpress actually becomes cheaper than many of the fastest SD cards, so the balance is definitely tipping in CFExpress' favour where you need >250MB/s write speeds.
What I'm saying is that you can still go a very long way with SD cards, and this is where Apple is positioning its offering - for the majority. At the top end, people with $10,000 stills cameras and >$50,000 cinema cameras will not be bothered about buying a separate card reader.