It may seem radical, but Sony should force all its studios to use the Decima Engine, I don't see anything negative about that
As happens in Ubisoft, even if Sony has different engines, they share internally their tech and tools, even with their 2nd party teams and in a few things with all PS 3rd party devs.
Some engine team may come with some improvement for lighting, other one for shadows, other one for handling loading or whatever and once implemented the other ones adopt it when it fits.
Also, depending on the needs some engines have specific stuff tailored to the need of their games while doesn't include some other modules that won't use to optimize. As an example, Polyphony may have stuff specific for cars/racing games and VR, or multiplayer, that the other ones may not have.
And well, some devs are used to their engine and prefer to don't need to change to another one.
So to have different engines has a positive side.
But also means that a lot of work is being made multiple times for multiple engines, and that if someone moves from an internal studio to another one will have to take some months to learn the new engine.
It would be better that in the future all engines would become more similar to each other in usage and internals, eventually becoming a single engine with the best parts of each current engine. It would also help them meaningfully reduce development costs, something they'll need to do. But as of now their current approach is ok.
It would end up being monotonous. Although engines can change drastically in looks game to game but still.
The artstyle and game design define how games look and feel.
It's up to their devs if two games look or feel similar or completely different, not to the engine.
Even if they use Unreal Engine, they can change specific parts of their game or engine if desired instead of using the default or popular ones.
No one would have for saw that that goes without saying. Either way your engine should be capable of worst/best case scenario. Especially when they know it's a GaaS game. WTF thy thought only 5,000 ppl would play. That's short sighted thinking and I doubt they thought something like that. Shit just happens.
Helldivers became a success as a PS3, PS4, Vita and PC release. Some time later they started to work in the sequel with AAA ambitions thought in the next gen.
So at some point they saw that engine they were using wasn't enough. Regarding sales/user expectations, being a AAA game they obviously thought were going to be bigger, maybe 5x or even 10x bigger. I mean, around 10x times bigger would be the biggest PS Studios games.
But hey, that's one thing and another one that they'd go from a small Vita game to a monster which would perform (as of now) 68x times better in Steam CCU all time peak, outperforming all games from Sony, Bungie, MS, Take 2/Rockstar including GTAV, EA (with Apex Legends as exception), and Activision Blizzard (with CoD as exception) and most other publisher or even indie hits like Among Us.
Obviously Arrowhead and Sony saw potential in the idea of turning Helldivers into a 'smallish AAA' game but nobody could expect such success.
