Smart contract code is open-source[1], so the centralization you're talking about shouldn't be confused with the (de)centralization of power people talk about when they talk about the decentralized web or decentralized finance. Circling around a small set of trusted open-source technologies doesn't cause centralization of power, as we can see with GNU/Linux, HTML/JS, ...
[1]: On many platforms smart contracts are stored in the form of high-level interpreted languages. On Ethereum, the blockchain stores EVM code (assembly), but contracts that haven't "verified" their source, typically by uploading the high-level code to etherscan.org, are seldom used (with some notable exceptions).
I contend that it doesn't cause centralisation of power, but centralisation and power are inevitable should the project hit the mainstream, proper:
- GNU/Linux - GNU/Linux is open source but centralised. The userspace is the part that is distributed, via operating systems, and the source control is distributed, via git. But it's all for one Linux kernel. You can also build your own kernel, but that doesn't really make linux 'decentralized'. Similarly, Linux for a lot of people means 'Ubuntu'.
- HTML/JS - this is centralised under WhatWG/W3C, etc. Arguably, these days, it's actually centralised under Chrome, because what Chrome does eventually becomes the spec. You can freely build your own implementations of HTML and JS/Ecmascript but most likely, you are using the centralised implementation via webkit, blink, or gecko.
So, the fact that smart contract is open-source doesn't really mean anything. It'll grow big, then as a matter of convenience it will start to consolidate. git, for example, is decentralised, but git forges (github, gitlab, etc.) provide centralisation as a convenience.
[1]: On many platforms smart contracts are stored in the form of high-level interpreted languages. On Ethereum, the blockchain stores EVM code (assembly), but contracts that haven't "verified" their source, typically by uploading the high-level code to etherscan.org, are seldom used (with some notable exceptions).