Right now, you get diminishing returns on ships Mk. III and up with both utility per resource cost AND utility per knowledge cost.
Are you saying that, even though one xor the other is fine, having both just seems unfair and trying to push "variety" too far to the detriment of the game?
Anyways, your point in that trying to enforce variety too much will actually lead to a different "sameyness" is very interesting.
Tactically, if everthing was equal and blanced, you would want a little of everything. This is because the ai, by its reinforcement logic, tends to get a little of everything. There is not much reason to specialize.
In the current game environment, things are not balanced, so you are tempted to sacrifce frigates to get better bombers, for frigates counter thing that are not gold like (smaller fleetships) while bombers threaten bigger, more threatening things (forcefields, forts, any large ship).
Since it is impossible to fully balance a game, there should be some additional incentive to be diverse. You could make advanced things more expensive, or make them cost more knoweldge. However, the current game environment does both HARD. Without an IV factory, the III's compared to II's cost at least twice as much in K and in resources for 50% more power. So compared to I and II and II to III there is less then 50% efficiency for either, 25% if you count both . BUT, don't forget the III's get less fleet cap, so it is even less then 50% / 25% efficiency.
50% efficiency in anything is almost always bad if things are balanced. 25% is bad even if balanced.
If you have an advanced factory, you get about 150% bonus things in bonus stats (III's and IV's) for 125% of the K cost. But those IV's cost three times the amount for 2x the stats. ***
***math here needs to be double checked.
So the end result is that the game counters specialization hard. But specialization is what makes games unique. Specialize in enclaves and cheap zerg units are unique. Specialize in starship only games are unique. Specialize in <whatever> is unique.But the current setup is a little bit of everything. The goal may be diversity, the end result is railroading and actually an increase in the same tactics.
For example, if you specialized in only one base fleetship, you would NEVER send them all on FRD.