I don't get how some PP think that fitting by eye is LESS skilled than taking measurements.
The best fittings I have had have been by eye, in small shops that sold a small but varied range of brands.
If a fitter is less skilled then they will have to resort to using a tape measure and then, being less skilled, they will also be less likely to recommend a bra that is the right shape and fits well.
Unless M&S are going to employ dedicated bra-fitters who are able to gain enough experience to acquire the necessary skills to fit by eye, their bra fittings are unlikely to improve.
However, if women have had bad experiences with M&S bra fittings it is probably at least partly due to the fact that M&S does not have a wide range of brands to choose from in store, ie. rather than a wide range of M&S-designed own lines.
I learned a very long time ago which brands built bras for my shape and which ones did not, irrespective of bra design. When I have been tempted now and then to try a bra from a brand that has proved unsuitable in the past, I have quickly discovered that it was not worth the time and effort.
Women who are very happy with M&S bra fittings are likely lucky to be the shape that M&S own-line bras are designed to fit, irrespective of style. Unfortunately, I have found that I am not that shape, so even the best M&S bra-fitter is unlikely to offer me a bra that fits me well and that I like the look of. C'est la vie 🤷♀️