It seems to me like you are missing the point. The advantages that BU brings are primarily political, empowering the participants of the Bitcoin network. From a purely technical perspective BU is not that different from Core or from the protocol today as a whole. Considering that you are not interested in discussing the political implications only further proves to me that you are completely missing the point.

That you think that a flexcap is similar to BU in regards to the blocksize only further highlights this misunderstanding. Flexcap if introduced by Core is a form of centralized economic planning, which history has shown us is ill advised. Flexcap will be charging miners for creating bigger blocks, this can be seen as a type of "taxation" or "fine", which would then be payed out to the next miner if they create a smaller block, forever adding a new incentive structure to Bitcoin mining in order to keep the blocks small fundamentally changing the economic policy of Bitcoin.

Almost all of the blocksize proposals so far have been forms of centralized economic planning. Since we are free to choose alternative implementations, these alternative have their own proposals and policies. We are limited in our choice however by the options that are giving to us by developers. This is like a form a representative democracy, with some of the accompanying problems that this creates. Singular implementations must decide on the vision and path for Bitcoin into the future, this is where some implementations have now diverged. Bitcoin Unlimited in many ways is different since it has transcended this debate, both small blockists and big blockists can arguably use BU, instead of dividing up into political camps and fighting over what the vision of Bitcoin should be, which would most likely lead to a split. Many of us can choose instead to allow the free market to decide what the blocksize should be in a more distributed and emergent fashion. Bitcoin Unlimited helps to further remove any barriers to this free market. Bitcoin Unlimited is also about disempowering developers from having a disproportionate influence over the economic policy of Bitcoin.

I can see why you might think that developers having more control over the protocol is a good thing. I can bite the bullet and say that this would most likely also slow down development, as I did in my conversation with Greg Maxwell. This can be percieved as being a positive thing however, making development in Bitcoin more conservative. It can also be argued that many developers are not well equipped to decide on the future of Bitcoin, considering that many of them have engineering and computer science backgrounds, these questions have a very large scope where the humanities also become important. I would argue that it is better that the market decides, I think that the wisdom of the crowd will always be better compared to any other form of centralized authority deciding for us. We can only lose our freedom by allowing others to decide for us.