I wore only hiking sandals (took two identical pairs, partly to wear a clean pair in public areas of guesthouses/ hostels, partly to swap as patterns of wear etc make this helpful) on a five day walk from Bregenz to Rapperswil in Switzerland last summer. Only about 105km but obviously lots of up and down and unpaved hiking tracks.
The only annoyance was the gravel paths where small stones sometimes got on my sandals, but even on those the relief of cooler feet in comparison to closed trail runners or hiking boots was worth it!
I have (separately) done 30km hiking days on trails in hiking sandals. In hot weather they're so pleasant to walk in (and quick to take off to paddle in mountain streams - or indeed if you take two pairs and the streams are stoney fine to wear in the water).
There are sandals and sandals obviously, but it's certainly not true that trainers are universally better for walking in.
For me hiking sandals are multiple times better for serious hot weather walking / hiking than any trainer or running shoes - only my actual hiking boots are better for walking (but my feet swell in them in very hot weather and actually get waterlogged and look as though I'm getting trench foot by the end of the day, and if I start in boots because of a serious rocky climb the relief of switching to hiking sandals for the rest of the hike is immense.
At home on a work day I do between 10,000 and 15,000 steps (active job) and again wouldn't want to wear anything but hiking sandals most of the day (sometimes change to closed indoor shoes for specific tasks but that's a very small part of the day).