Node runner is the enforcer and they are free to run what kind of software they wanna run, it's consensus, just because you developed something doesn't mean it'll get adopted automatically.

You have stated that "Bitcoin's security depends on the behavior of software maintaining consensus." and it's exactly that, it depends on software maintaining consensus, you've actually answered the question yourself.

Developer develops, node runner vote. In simple layman terms.

From your OP (above), you've clearly highlighted the division of responsibilities, which includes:

- Node runners
- Consensus
- developers.

Node runners: This part is responsible for transaction verification and record keeping. They may include any single individual—including you—who has a good working system with enough storage, and an Internet connection.

Consensus: This is an automated system that determines how a transactions is approved. It includes a set of rules that the whole system follows to approve, verify, and store a transaction. This part includes the following:

- Miners: they pick up pending transactions from the mempool and group them into blocks using high computing power, earning rewards based on first come basis—it's basically a competition of computing power out there (proof-of-work).

- Nodes: they verify the sorted out transactions as valid—ensuring no double spending—and eventually keeps a record of them.

Developers: this is the last part of your OP. They actually propose new rules that can be implemented by the system above (nodes and miners). Lemme explain further.

When a new rule is proposed, it is sent to the Bitcoin devs, the whole Bitcoin dev community look into the proposal, and either accept or reject it. If accepted, they implement the code in Bitcoin core, the code will be triggered by a condition—normally, an "if" statement (the newly implemented code is usually strictly applied, in that, the previous rules are also wired in the new implementation)—although, sometimes there could be a hard fork.

After this process, it is left for the miners and node runners to download this new version and trigger it. If the community doesn't like the new implementation, they can simply ignore the trigger condition and continue with the previous rule.

No one arm can dictate the eventual rules to apply without a general and synchronous agreement—you can't enforce a rule, or make changes independently. This is why the protocol is actually decentralized despite devs being the ones that write the rules into the code base.

Let's walk down history lane. Gavin Andresen was one of the core developers of Bitcoin who worked closely with Satoshi. Satoshi even said He has left things in the good hands of Gavin and everyone in a private email to Mike Hearn which was later made public—these are historical facts you can easily find anywhere.

The above statement is to show how influential Gavin was in the Bitcoin project—you can Google or wiki him to know more. Yet, when he proposed BIP101 to increase the block size of transaction from 1mb - 8mb, it was rejected. He had to go create his own coin. Even when a similar proposal was made by miners and some devs in 2016, it was shut down using UASF (Users-Activated Soft Fork): this activated segwit based on nodes influence.
(source)

The brief history above shows how dev—even though very influential, and can write and propose new rules—cannot enforce these rules and have centralization of authority over the network.

There are developers, miners, node operators and users that make sure the Bitcoin ecosystem is running, but that does not mean total control or centralization. You could say that they manage the Bitcoin ecosystem without a direct control over it. Developers write the software and could write certain changes to Bitcoin core but that would not pass if the nodes and users are against it, that shows that it's not run by a central authority but collective contribution of all parties mentioned.

Devs write code, they don't control what people run. Core has no auto-update, so if they pushed something shady nobody would be forced to install it.

The blocksize war is the best example where big companies and miners backed SegWit2x (New York Agreement), and it still died because the economic majority refused to go along. Decentralized doesn't mean nobody has a louder voice, it means nobody can force the rules on you.