Did I miss something?

Doesn't look that way.

Note that iMac Pro memory is not user accessible. If you think you may need more memory in the future, consider upgrading it at the time of purchase.

https://www.apple.com/shop/buy-mac/imac-pro?product=MQ2Y2LL/A&step=config#

iMacs have always been limited in what can be upgraded.

iMacs always had NO user serviceable parts inside.

For a few years, 21 inch screen iMacs had NO user upgradeable components, at all!

It would appear Apple is staying with the iMac tradition with the new iMac Pro.

You need to order all the available options (CPU, GPU, and RAM) at the time of purchase.

The difference is you are picking much more expensive component options in the iMac Pro line.

If you cracked opened your 2007 iMac, this must have been after the warranty period had past as NO iMac made since 1998 has ever had user accesibility to its innards.

If you would have cracked that iMac open in 2007, you would have voided ALL Apple warranties AND voided ANY servicing of that iMac by Apple stores or Apple in CA.

All iMacs are non-ugradeable (except RAM) to the end user.

Has ALWAYS been this way.

If the iMac is out of warranty and you decide to crack it open to do anything inside, you automatically void any later help from Apple or Apple Stores.

You might get help from an Apple authorised service center OR any other third party Apple repair center, but Apple, itself will service a Mac that has been tampered with by the user.

You end up being on your own with any Mac repairs you cannot fix yourself.

Let's put it this way: just because you can probably "crack open" any iMac - is it supported by Apple? NO, it is not. That means: if you do, you void your warranty and Apple may not repair it as it may determine that the problem was caused by a third party component. So, Apple has decided that iMacs are not user accessible. And, although I think OWC is a great place, they are NOT Apple or an AASP, so just because they have instructions on how to, following them is not "blessed" by Apple. Here is an excerpt from the hardware warranty:

This Warranty does not apply: (a) to consumable parts, such as batteries or protective coatings that are designed to diminish over time, unless failure has occurred due to a defect in materials or workmanship; (b) to cosmetic damage, including but not limited to scratches, dents and broken plastic on ports unless failure has occurred due to a defect in materials or workmanship; (c) to damage caused by use with a third party component or product that does not meet the Apple Product’s specifications (Apple Product specifications are available at www.apple.com under the technical specifications for each product and also available in stores); (d) to damage caused by accident, abuse, misuse, fire, liquid contact, earthquake or other external cause; (e) to damage caused by operating the Apple Product outside Apple’s published guidelines; (f) to damage caused by service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (“AASP”); (g) to an Apple Product that has been modified to alter functionality or capability without the written permission of Apple; (h) to defects caused by normal wear and tear or otherwise due to the normal aging of the Apple Product; ℹ if any serial number has been removed or defaced from the Apple Product; or (j) if Apple receives information from relevant public authorities that the product has been stolen or if you are unable to deactivate passcode-enabled or other security measures designed to prevent unauthorized access to the Apple Product, and you cannot prove in any way that you are the authorized user of the product (eg. by presenting proof of purchase).

I know ALL about OWC (macsales.com)

Macs out of warranty can be broken into, at that time

You can purchase LOTS of things from OWC for Macs and iMacs, MacMinis, etc.

You can do ALL the DIY stuff yourself once the Mac is out of warranty.

But if you choose to crack open any iMac after the warranty period is up, that's fine!

Just don’t expect Apple to service any Mac once they have been cracked open and modified by the user with third party components.

Apple will NOT service Macs, that aren’t supposed to be tampered with by the user, but was tampered/tinkered with by the user and the user went and installed non-Apple third party hard drives, optical drives SSD kits, etc..

3rd party RAM is usually the only exception to Apple's servicing rules.

If you have an iMac that was altered with 3rd party hardware components and something like the logic board, GPU and/or Power Supply dies, or the screen fails, don’t expect Apple, itself, to do the servicng/repair of that Mac.

It's that simple.

I am just pointing out that the iMac line, since 1998, has ALWAYS been an “average user/consumer/computer novice” Mac that Apple purposely designed so said consumer could NOT tinker with anything inside of an iMac.

iMacs were conceptualised and designed as a mostly sealed-up computing appliance.

The 2012-2015, and to some extent, the new 2017, 21.5 inch screen iMac models are STILL nearly sealed up, glued up computing appliances, not to be tinkered inside with by the end computer user.

The iMac line HAS ALWAYS been designed like this!

You stated you are a long-time Mac user.

You do not know this of the iMac lineup through the last 20 years?

As far as your OWC statement is concerned, I buy plenty from them, but all of my upgrades for my late 2009 iMac, with the exception of replacing the RAM with OWC RAM, everything else dealing with my iMac is all externally connected hardware.

I am, currently, running my iMac from an OWC externally enclosed 480 GBs SSD running over FW800.

Believe it or not, my iMac feels like a new Mac running an external SSD!

The speed boost/response time in performance IS really noticeable!

I have 3 other standard, 7200 RPM hard drives I also purchased from OWC over the years!

These are all STILL doing well!

Thank You for that actual info excerpt from Apple.

😉