I think the common idea that people have a finite capacity for X is reductive, not really the whole story... to put it mildly.
I think that's at the heart of the concern expressed by behavioural scientists (sorry, Oli). I've just been down a brief rabbit hole, and as far as I can work out, Michie (quoted above) is concerned more with the term, and it being not something used in her field, and an over simplification of a number of complicated factors which are addressed in her field. Some of the concerns can be seen here (in which she's quoted): https://www.theguardian.com/world/2020/mar/13/behavioural-scientists-form-new-front-in-battle-against-coronavirus
What's worth noting, though, is that they also say: "What studies we did find, such as studies on the continuation of protective measures during past epidemics such as H1N1, seemed to indicate that yes, compliance with such behaviours tailed off over the course of an epidemic, but it is unclear that there is more at work here than the fundamental, well-established principle that uptake of protective behaviours is moderated by perceived threat." (which echoes, I think, what Michie says in the Guardian piece).
That is all to say, I think you're right - the issue is more with an over simplification of the idea, rather than something (i.e., a number of factors) akin to fatigue not existing.
I think that's at the heart of the concern expressed by behavioural scientists (sorry, Oli). I've just been down a brief rabbit hole, and as far as I can work out, Michie (quoted above) is concerned more with the term, and it being not something used in her field, and an over simplification of a number of complicated factors which are addressed in her field. Some of the concerns can be seen here (in which she's quoted): https://www.theguardian.com/world/2020/mar/13/behavioural-scientists-form-new-front-in-battle-against-coronavirus
Additionally, you can see direct criticism of the term from behavioral scientists here: https://behavioralscientist.org/why-a-group-of-behavioural-scientists-penned-an-open-letter-to-the-uk-government-questioning-its-coronavirus-response-covid-19-social-distancing/
What's worth noting, though, is that they also say: "What studies we did find, such as studies on the continuation of protective measures during past epidemics such as H1N1, seemed to indicate that yes, compliance with such behaviours tailed off over the course of an epidemic, but it is unclear that there is more at work here than the fundamental, well-established principle that uptake of protective behaviours is moderated by perceived threat." (which echoes, I think, what Michie says in the Guardian piece).
This study also addresses time as a factor which can have negative outcomes (not just in terms of adherence) : https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30460-8/fulltext
That is all to say, I think you're right - the issue is more with an over simplification of the idea, rather than something (i.e., a number of factors) akin to fatigue not existing.