1. There is no guarantee one of these 'uncontroversial' hard forks won't become controversial, especially as the community gets larger and there are more voices. What was once a consensus position can become contentious due to new information or ideas. This makes the market (people involved in the Bitcoin space) uneasy.
2. Hard forks centralize the community, because in practice, they require the community to coalesce around a small ad-hoc governing structure for the hard fork to have consensus. This makes frequent hard forks antithetical to the decentralized quality of Bitcoin.
Thats an evolving quality, IMO. And there is nuance. The theoretical / idealized architecture of bitcoin has a decentralized quality, but in practice, it does not. Mining is centralized by pools. Consensus is centralized (by which chain the fiat -> bitcoin onramps decide to use.)
The original design mentioned nothing of how the code would be maintained, save for the concluding line of the whitepaper "Any needed rules and incentives can be enforced with this consensus mechanism." And of course the consensus mechanism described is "They vote with their CPU power, expressing their acceptance of valid blocks by working on extending them..." My point being that while the network architecture was designed to be decentralized, the human component was not addressed. The human-code-management component.
As the OP states, a dynamic block size seems the most logical way forward.
The goal of cryptocurrencies is to remove humans from as much of the equation as possible. An ideal cryptocurrency will continue functioning as long as people mine it (and the internet works).
Not really. A protocol can remain static while overlay protocols are built on top of it, and technology is created to interact with it. A decentralized consensus protocol needs to be unchangeable for the sake of stability, immutability, and censorship resistance, IMHO.
Indeed, it can remain static, and it will eventually become locked in. At some point it will be locked in, and we'll be stuck with whatever was last agreed upon.