It would be great if all mappers always added surface, smoothness, width, etc to paths but this is far from reality. Across the four main path-like primary tags (footway, bridleway, cycleway, and path), surface is the most prevalent property key at around 30%. So around 70% of paths have no surface tag. For smoothness and width the missing percentage is more like 95%. Even if the majority of paths did have these property tags, data consumers would still have to decide what to do with paths that don’t have them. With no agreed upon default, data consumers each make different decisions about what they think the default is. Different map renderings then drive mapper behavior, and the data becomes as inconsistent as the varying interpretations.
Some sort of documented default meaning would improve the situation, but it can’t be too specific. I agree with @Peter_Elderson that a conservative default assuming a propertyless highway=path is probably a rather rough, undeveloped trail is best. Assuming a highly developed, accessible path where there is actually the opposite is a worse outcome. Adding more specific property tags in both cases is great, but I firmly believe we will never reach 100% coverage of supporting tags. There will always be some paths mapped with just one primary feature tag as a first pass.