The only thing wrong about the 3.5 is that it's not very favorable from a tax point of view, in certain countries with displacement tax it just doesn't work.
From a technical point of view there is no such thing as the "golden number of 0.5L/cylinder". That number is only favorable when you also consider regional policies, although conforming to regional policies alone is a good reason to go for this number. Toyota/Lexus already paid the price for not sizing their engines for displacement taxes in the past and it looks like they didn't get the memo.
In fact, there is a very good reason to have as much capacity per cylinder as possible. Modern engines are always designed around maximum efficiency, and thus require very complicated designs of the combustion chamber, the pistons, the valves, the fuel system, basically everything. These designs are much easier to achieve in a larger cylinder, since the extra volume allows more margins during manufacturing. When you incorporate complicated designs in a very tight dimension, you will have problem with manufacturing. The large cylinder also gives more space to allow for tumble flow that is one of the key aspects of achieving 40% thermal efficiency on the Dynamic Force.
The A25A four cylinder has 0.625L/cylinder, which is also shared by the similarly wonderful Mazda 2.5 Skyactiv. In fact, Toyota even showed how the 0.625L/cylinder A25A has slightly better tumble flow and heat release rate than the 0.5L/cylinder A20A on their own website.
https://newsroom.toyota.co.jp/en/powertrain2018/engine/
Automakers always try to go for maximum capacity per cylinder. Even BMW itself made the choice with the three-cylinder B38. Yes it is a 0.5L/cylinder but that is a lot for a three cylinder. How much capacity per cylinder you can get is related to the inherent balance of the engine layout. When you have a very smooth engine, you can basically get very high capacities since the vibrations don't add a lot of stress. That is why you see Inline-6s and V8s in high capacity engines for commercial and military vehicles. For an Inline-3, the vibration adds so much stress that a 1.5L needs a lot of extra reinforcement and vibration dampening. The auto industry once considered 433cc/cylinder (1.3L) was the upper limit for a practical Inline-3 engine, which is what GM settled with. But BMW (and Ford) still went through all the trouble to make that 1.5L because it is just more efficienct than a similar 1.5L Inline-4.
The weight part is just non-sense. In fact the 3.0L, 3.5L, and 4.0L GR engines all have the similar weight (at ~170kg). There is no direct correlation between displacement per cylinder and engine weight but it's all down to design specifics and materials choices. The LFA's 4.8L V10 is lighter than the 3.5L V6. The 6.2L LT1 is lighter than the 4.4L N63. Also, power density is rarely the main objective for engine designers otherwise everyone will be building OHVs or rotaries; there is a weight penalty for trying to be efficient.
I suggest that you actually do some reading on Dynamic Force and try to understand how engineers made their design choices. I don't think calling the engineers responsible for making the second (or maybe third) best engine in the world uninnovative makes you look any smarter.
You simply missed the point of why the V35A is an awkward engine. The engine itself is technically sound, but it just doesn't play well with certain policies of certain governments and that is the core of the problem.
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