What you have done is justified 'everything under the sun' so long as it technically operates in a free market. But the ability for users to switch does not guarantee that the incentives to compete are at all healthy or robust.
You are correct when you say centralization affects users indirectly and they will simply use services which are most immediately convenient. But competition is not giving users much benefit because every centralized service has a monopoly on their instantiation - it's not like you can make a few tweaks and give everyone a moderately better experience - you have to start from scratch and make yet another siloed and extractive platform for any and every improvement. And then if you do they can easily copy you back before you build a fraction of their momentum.
X isn't going to let you improve the experience and just take users; they're going to say: "Have fun building up user trust and security infrastructure - also you're never getting our users." That's the difference between a protocol and a platform. The reason companies build platforms is because they need to fund infrastructure and opsec at scale so they effectively need to build monopoly protocols i.e. 'platforms.'
So "switching is easy" is meaningless. Building successful competing platforms is, by design, very difficult; it takes large investments and huge risks and a lot of rebuilding what's already been done just for the sake of catching up to platforms who have obvious incentives and built-in methods to discourage competition via their tight, centralized structure. Even if the platform is better, it will probably fail relative to its predecessor.
Decentralization most directly helps builders. If the basic requirements of a service are sufficiently decentralized security, networking, front-ends, then a builder who wants to compete via small (or any sized) improvements *does not need to rebuild the entire service.* Small builders who would otherwise not have access to startup capital, risk tolerance, or excessive build-hours would be equally able to compete because their decentralized access to the basic requirements cannot be locked behind an extractive economic scheme.
Imagine every talented programmer making open source software could leverage it atop secure, robust and interoperable networks. And they could earn money from it.
There is a massive difference between being a passive proponent of the free market and a maximalist for market competition. Your justification for the state of the field is a passive retreat to free-market capitalism. Users certainly have the ability to choose, but there are all sorts of schemes and situations in a free market where competition is choked. Honest proponents of decentralization are maximizers of opportunities for competition - they recognize that the free market is a gradient and not some binary quality which automatically imbues every operation inside it with good accountability.