>I think that, for a given amount of effort (hours and study intensity) you’re willing to spend, everybody has a ceiling that they can reach.
I disagree with your disagreement, because simply controlling for total studying time and intensity is too reductionist. Different players have different sticking points when it comes to chess, e.g. weak strategic planning, weak tactics, poor positional understanding, bad endgames, etc. Your implicit assumption is that most players at some playing strength, are at that playing strength in all aspects of their game. In practice, that's simply not the case for many.
To give a concrete example, my classical rating on lichess hovers around 1800, but if you look at my tactics puzzle rating it's well above 2000, suggesting it's the positional and strategic aspects of the game that I'm weak at, which anecdotally feels true based on how I both win and lose most of my games. If I were to get a coach or deliberately work on those weaker aspects of my game myself (something I have not done), so that they're no longer the bottleneck of my performance, I could very well break this rating plateau I've been stuck in for the past half decade or so, and shoot up another 100 or even 200 points. I also have a friend of similar strength level, who has the opposite profile as me: strong positional and strategic understanding, weak tactics. And despite more or less an even record, whenever we play against each other, his wins are almost always grinds, while my wins are usually some tactical shot he missed or blundered into.
The bottom line is, at my strength level, and I'd hypothesize even up to the low to mid-2000s rating levels, these unbalanced types of players are probably more common than balanced players with similar ratings in all aspects of their game to their overall rating. The latter kind, you might be able to argue, have reached their natural ceiling; but even here I'd be surprised if they cannot improve more by deliberately strengthening aspects of their game. Conversely, based on my experience of 10+ years playing chess regularly, the vast majority of players simply don't have a good understanding of their own weaknesses. Many unbalanced players like myself, with the correct type of training and practice, even if total time isn't too much, can absolutely make significant improvements to their overall performance.
>If what you say is true, why hasn’t Magnus Carlsen reached ELO 2900? Lack of deliberate practice? ELO deflation requiring players to get better to keep the same rating?
In the case of top-level IMs, GMs, and certainly super GMs, who don't have glaring weaknesses in any aspects of their game, it's likely the case that they indeed did reach their ceiling. But these are the only people I'd be at all confident in making such claims.