I think its a matter of ongoing debate online about what is the "correct" way to code curbs tags on these smaller crossings. From what I've gathered the most important thing is to never have the curb tag on the main sidewalk way itself (as that would suggest that you have to step over a curb to continue down the sidewalk). 

Typically in cases where the sidewalk is wide enough to support a 90 intersection between the two sidewalks, I separate the curb tags (similar to the graphic shown in https://wiki.openstreetmap.org/wiki/Key:crossing or like node 5765177201 if there wasn't any grass between the two curb tags), even if the curb is in reality one large curb let down. I usually only do the nubbin for those crossings where the curb let down is directly on the apex of the sidewalk curve and right after the stop sign (as the crossing shouldnt be before the stop sign). That way its off the main sidewalk way without having the sidewalk way mapped overtop of the asphalt areas.

One general comment I have seen online against the nubbin is that it can create additional routing instructions, which is a fair point, although I personally support the maps being more realistic than abstract.

With regards to your comment about bypassing the curb I agree that could be possible. However, you could make the same argument for those cases where the curbs are physically separated in real life. If one of them is a high curb, someone could theoretically follow the curb along the asphalt roadway to reach the other crossing. As we don't account for these edge cases for larger intersections where it would require another separate link between the two, I  am somewhat less worried about it in the cases of smaller intersections (if that makes sense). If we wanted a truly 100% accurate representation for a freemoving pedestrian, all the sidewalks and crosswalks would be pedestrian areas haha.

Really I think that at the very least having the presence of a potential curb on OSM is worthwhile as that can allow additional information to be added through apps like StreetComplete.