Question: Is there a word or expression in English to describe someone who has the power/ability/skills to act upon something, but we already expect them not to do it because they see themselves as not in a position where they *need* to take an effort to act?

It doesn't necessarily refer to a person, could be a corporation as well (and quite possibly most uses of this word/expression would actually refer to companies I believe). It is somewhat related to an adage that says "the difference between the rich and the poor is that the rich is allowed to make blunders for a longer time." But this would be its reverse logic, in the lines of "since I am the rich, I will not make anything at all, why should I bother?"

Two examples of where this word/expression would fit:

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> The *X App* has a serious problem of user experience that could be greatly improved with a few simple features, but since they have virtually no competition, they [are] \*\*\* and we'll never see those things implemented.
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> *Company Z* should really step up their game, *Franchise W* has become stale for over a decade now, but since the fans keep coming, they [become] \*\*\* and just rehash the same thing over and over again.
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"A is not compelled to do B" is an expression that might come to mind, but I'm not sure if it could convey what I am trying to present here.

Answer: 
The following idiomatic expression can be used to convey the idea you want to express:

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> [rest on your laurels](https://dictionary.cambridge.org/it/dizionario/inglese/rest-on-your-laurels)
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> to be satisfied with your achievements an not to make an effort to do anything else.
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(Cambridge Dictionary)

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> * *The X App has a serious problem of user experience that could be greatly improved with a few simple features, but they are resting on their laurels and we'll never see those things implemented.*
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The situation you describe of some people (or companies) being neglectful or lazy, and not caring to act because their (perceived) dominance may come from a bias known as the [overconfidence\_effect](https://en.wikipedia.org/wiki/Overconfidence_effect)

The dictionary has many terms for not acting. I like Indolence, with its adjective

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> [**indolent**](http://www.websters1913.com/words/Indolent)  
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> indulging in ease; avoiding labor and exertion; habitually idle; lazy; inactive.
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This is the same to say to someone that is [neglectful](http://www.websters1913.com/words/Neglectful) or [careless](http://www.websters1913.com/words/Careless)

So, one way to refer their lack of action caused for some perceived comfort is to say that they are behaving like

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> **overconfident careless** people
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