We could both be interested in bitcoin as a tool to take wealth from one another... For instance, I may want bitcoin to fail...
Bitcoin is just a piece on the chess board, in the game of taking away wealth of your peers into your pocket ; the only "common interest" we have in such a system, is to rip off one another. This is exactly because bitcoin (like most crypto) is a speculative asset, which is exactly that: a tool to rip off peers in essentially a zero-sum game. Whether that tool thrives or breaks when doing so, is of lesser importance.
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People who believe bitcoin has no long term future are unlikely to participate in the system in any sustained manner. If you believe bitcoin is just a scam you may participate in the periphery trying to make money but logic will dictate you constantly keep one foot in the doorway. You will hesitate at being involved in anything but the most superficial manner.
Probably you will choose not to invest at all but if you do then you are likely to cash out of the system at some point choosing to take your profits and leave the "tulip bubble" you have no confidence in. In this manner those who do not appreciate the underlying consensus mechanism will be diluted and replaced over time by those more interested in long term cooperation and building value.
What you say doesn't contradict what I'm saying. You may claim that there is a hard core of "true believers" which ARE indeed a community. Most probably, yes, there is probably a hard core community of true believers. However, the questions are:
1) how much of the ecosystem do they make up at any given moment in time
2) what influence can they have on the ecosystem ?
The way bitcoin is designed to be, turns around antagonism: we all want to possess coins, and if you possess a coin, I don't and vice versa, which makes us de facto antagonists ; there is competition in proof of work mining and the gains that go with it ; and deep down, it turns around speculation (which is a zero sum game). There's nothing in bitcoin that is designed to be cooperative, apart from having to stick to the consensus protocol for reasons of greater loss if you don't. This is entirely different with an open source software project, where the participants - the creators, devs, users... are not put on antagonist relationships and each can profit of the contribution of another one. There are many other systems that have an intrinsic functioning that lead to altruism ; but bitcoin and many other crypto are fundamentally based upon an ecosystem of antagonists. In fact, if everyone were fully altruistic in the bitcoin eco system, it wouldn't make any sense to use it !
As such, it is very well possible that there is a small core of hard believers in bitcoin that are perfectly altruistic and form a community, but most probably they have almost no influence on the power games and the antagonism in bitcoin: they mostly don't control large hash rates, they don't control large capitals that determine market price. These are mainly aspects in the hands of "true players of the game", that is, antagonists trying to get wealth out of one-another's hands.
You are right that most probably, this core of believers is steady in time. However, at any given moment in time, this core makes up only a very small fraction of the eco system. This means that the "non believers" are of course coming and going, and are not the same crowd at different moments in time, but are nevertheless the influential majority at any given moment in time.
In other words, to influence bitcoin, you need big money (whether in the market or with hash rate or with developers) ; if you are in bitcoin with big money, you're there to use it to get more money (out of the pockets of other players). And if you do that, you're most probably just a temporary, but important antagonist in the system, and these constitute, at any given moment, the power structure of the system.