Women have different body proportions compared to men as well as different shapes. One of those differences is leg length compared to torso which is longer in women so on average a 5'10" woman would have longer legs than a 5'10" man. I'm 5'10.5" ish and have a 34" inside leg if I try on men's trousers in flat shoes.
I gave up trying to buy in M&S due to sizings which would require me to chop my feet off to reach my shoes, even with the much vaunted "longer" trousers. I tend to agree that trousers a bit too long can be altered, too short cannot. However more than a couple of inches and you end up with proportion problems.
The problem is not that Marks have changed one of the only two lengths available - the problem is M&S think that women of all shapes and sizes come in only two leg lengths whilst men have a variety of leg lengths available for each size option. They get away with it because we grumble but made do and still buy clothes which don't fit well, where men are likely to complain more and go elsewhere.
I've also found that whilst M&S seem to have woken up a bit on style their quality hasn't improved - especially for work wear. £40 trousers are not good value when they wash and wear so poorly.