The new machines are better. I'm not an expert, but I think the biggest differences are: sensors to stop adding water when it isn't necessary, a really powerful spinner in the washer that dries the clothes better, powerful spinner in the dryer. Btw, my washing machine is top load, fwiw.
Oh, and to answer a different question: why didn't we have this stuff years ago? I think, no one cared enough to do it.
The old top loader basically filled up a big tub with hot water and sloshed it around a bit.
Modern front loaders just incrementally add the amount of water needed as determined by weight. Presumably wouldn’t have worked nearly as well with the available detergents of 50 years ago.
Secondly, while I never used a top loader, from everything I've read, they are worse in every dimention, so the comparison seems inappropriate - you are introducing extra variables.
Also I am not clear why 'incrementally adding water' was not possible previously, People built a 'smart' toaster 70 years ago, that could sence how cooked your bread is, and it was purely mechanical:
https://www.theverge.com/22801890/sunbeam-radiant-control-to...
Added: And toasters still do a crap job of figuring out done-ness.
Did we discover new phisics of washing? Do we have any reason to believe further improvement is possible?
This is a physical process, you can't have infinite improvement - there is a certain minimum amount of water required to dissolve detergent, and it can be easilly calculated.
They don't do much, does anyone have a real explanation without any handwaving?