I’ve done some investigation, and these are my findings:
The new commander loading screen images are significantly different than the old. They have removed the additional units, but the older images also had significantly more transparency – furthermore, the commander was always centered to one side to give space to the units, so parts of the commanders’ bodies were inevitably cut off. (The new images also get cut off, but not in the image itself – the file itself has the whole body, whereas the older files were cropped to a greater extent.) The old images are actually smaller than the new images, and are more rectangular, wheres the new images are exactly square.
This change was done deliberately, with what appears to be significant work put into it, so I am skeptical of the idea that there were no good reasons for it. I’d like to see the old artwork back as much as anyone, but they likely removed the background units because they ran into limitations of some sort, or wanted to center the commanders more in the field of view (whereas before they had to be pushed more to the edges to give the other units space). And to add the other units back, it might not be as simple as just switching back to the old artwork – they’d likely have to create new versions of the artwork for every commander just to fit the current layout.
In addition, the Battle.net artwork created for newer commanders such as Zeratul, Stetmann, and Mengsk actually gives far more prominence to the background units, to the point where that artwork would actually not fit in the loading screen because the image is so wide, and the background units are so large. So they wouldn’t be able to use that artwork – they’d have to modify it somehow, or shrink the background units. For example, look at how large the Siege Tank is in Mengsk’s artwork; the Flame Trooper is nearly as large as he is. So there’s no way they’d be able to use that artwork without significant modification. It seems that no longer having to include the other portions of artwork gave them more freedom in designing the promotional artwork for subsequent commanders, although because that artwork is no longer designed with the loading screen in mind, it renders it effectively unusable in-game.
But there are some commanders who I think really would benefit from having some variation of their old artwork returned, even if it’s not for all of them. Swann in particular looks really out of place without his imposing Thor and Laser Drill behind him; it basically destroys the artistic integrity of the image not having them there, because I’m always aware that something’s missing when I look at it. I think Nova would really benefit from having her Griffon added back in. Zagara’s Banelings would be a fine re-addition (they’re pretty much part of her hero unit anyway); I think Karax should also have the Spear of Adun in the background as he once did, along with his Photon Cannon. Since Mengsk looks rather lonesome all by himself, I’d also appreciate it if they found some way to incorporate his Flame Trooper into his loading artwork. Stetmann is the same, so they could give him a Mecha Hydralisk.
In general, it depends on the artwork – but I think all commanders who are not themselves depicted as “combat-ready” should have an army unit or some other aspect of their kit included. Commanders like Dehaka, Fenix, and Alarak look fine on their own, but others – like Swann, Mengsk, and Stetmann – don’t look imposing at all by themselves. Karax and Abathur appear more imposing, but could still use the additions to display what tools are at their disposal (Han and Horner would also look pretty good with the rightmost background Battlecruisers added back in). In general, each commander where you can’t immediately answer “How does this commander fight?” should have some aspect of their army or offensive power on display.