An oligopoly is a situation where there are very high barriers to entry.

With Bitcoin, anyone can participate. The only thing resembling a monopoly situation is the core dev team, but that resemblance is only superficial: it's a natural monopoly for the moment, but the barrier to entry is not prohibitive since other, possibly even more capable software engineers could form their own team or even their own Bitcoin fork and it's possible that the majority would eventually support their fork.

This is the free-est market in the history of the world, because there is no coercion in it at all. It is entirely voluntary.

So we are faced with the seemingly paradoxical situation wherein we have a completely free market yet there is a natural monopoly development team. This means that, in the short term, the dev team has a lot of power and can seemingly maintain a uniform set of rules by "fiat."

But I think it's a myth that this ability to quasi-legislate is what underpins the uniformity and solidity of the Bitcoin protocol. The existence of this debate shows that many will not automatically follow the core devs (even if they were unanimous). Rather, the source of the uniformity is the belief each node has in the core protocol rules, with the hard BTC limit being the most staunchly supported. Adherence to the original protocol in all aspects seems to be supported as well, but to a lesser extent, and may be amenable to persuasion.

In other words, Bitcoin as a system has always been underpinned by the beliefs and wills of its participants, including the belief that the original protocol should not be changed lightly. The only way to make changes, then, is by convincing people to go along with them, and although the dev team may be in the best position to do that, there is no monopoly on persuasion.