Not irrational to be concerned for a number of reasons. Even if local and secure AI image processing and LLMs add fairly significant processing costs to a simple task like this. It means higher requirements for the browser, higher energy use and therefore emissions (noting here that AI has blown Microsoft's climate mitigation plan our of the water even with some accounting tricks).
Additionally, you have to think about the long term changes to behaviours this will generate. A handy tool for when people forget to produce proper accessible documents suddenly becomes the default way of making accessible documents. Consider two situations: a culture that promotes and enforces content providers to consider different types of consumer and how they will experience the content; they know that unless they spend the 1% extra time making it accessibile for all it will exclude certain people. Now compare that to a situation where AI is pitched as an easy way not to think about the peoples experiences: the AI will sort it. Those two situations imply very different outcomes: in one there is care and thought about difference and diversity and in another there isn't. Disabled people are an after thought. Within those two different scenarios there's also massively different energy and emissions requirements because its making every user perform AI to get some alt text rather than generate it at source.
Finally, it worth explaining about Alt texts a bit and how people use them because its not just text descriptions of an image (which AI could indeed likely produce). Alt texts should be used to summarise the salient aspects of the image the author wants a reader to take away for it in a conscise way and sometimes that message might be slightly different for Alt Text users. AI can't do this because it should be about the message the content creator wants to send and ensuring it's accessible. As ever with these tech fixes for accessibility the lived experience of people with those needs isn't actually present. Its an assumed need rather than what they are asking for.
I'm not really sure that's true. Labour has also downgraded its climate ambitions and ruled lots of necessary change out in favour of promising technosolutions. That means when change is needed the expectations have not been managed and we risk a megabacklash. The victory is dramatic and large but mainly due to FPTP. The victory is very shallow beneath the surface with lots of marginal seats and in lots of them Tory+Reform share is bigger than the labour share.
We can absolutely enjoy this moment but the big fight for climate I think has only just begun in the UK.