Alternatively, “desktop” implementations is just a closed laptop attached to a couple of ....
Well Apple desktops used to be upgradeable. They are not anymore or they have a very limited upgradeability, so in reality, they no longer fit the desktop model.
And about being a closed laptop - sure. And the trend in consumer sales is towards notebooks. And the ability of switchables that combine tablet and notebook functionality also effectively threatens notebooks without touch screens and pads that can't effectively compute. Apple refuses to be in that ever increasing market place so they are jeopardizing their future IMO, all to keep separate two business units.
Nonetheless if one wants things that take a while to perform, speed is important and if the computer takes some real time to complete its task, power is called for. A desktop will effectively work out of hours too. Many businesses are using notebooks merely as an interface to a workstation that does the work and which serves many employees.
For us niche apple users, the question could be does one buy a Studio and an Air, or instead buy a MacBook Pro Max? And being an expensive MacBook Pro Max with lots of RAM and drive capacity, it's sensible to protect it by putting it in a protective case. And have a more costly back up protocol and an insurance policy. And it's a bulky and heavy unit. A 15" Air does most things and is a lot cheaper than a Max, and it's a heck of a lot more portable. My 16" Pro M3 Pro Max with case felt huge compared to my 15.4" Intel MacBook Pro. Which is an argument for a separate desktop and a cheaper notebook. And IMO desktops last longer too due to less internal stress and less cooling capability, although with Apple desktops these days, they are not upgradable, so eventually they will loose their performance edge.
I suppose the lack of Apple's desktops to be upgradeable does make the Apple desktop a more threatened species.