The only reason i see them adding this diminishing returns is to force you to make more factories rather than constructors, like in PA.
However i do enjoy big numbers of constructors instead of factories because they are easier to harass than factories themselves, one of the things i enjoyed the most about TA was massive amounts of constructors switching around different factories.
Also it gives the player more control over what you want to build and how fast you want to build it instead of making more factories. Also its a gradual and smaler boost in production than you making factories, as it cheaper and faster to make constructors than it is to make factories.
Also the build power is mobile compared to static factories. I for example even enjoyed the massive swarms of air fabbers assisting factories, but that was a bit broken because you could instantly make defenses or proxy tier 2 facs and spam them at odd locations.
But since we only have land fabbers and they are kinda slow i dont really see why we should have diminishing returns per assist.
If anything the diminishing returns could only apply after a certain number of fabbers have been assisting but even then it doesnt make any sense.