If the soil for the turf area was prepped with a quantity of undecomposed organic matter that is the basis for your problem - it decomposed and resulted in subsidence. If doing the lawn over again don't mix in a bunch of organic matter that will cause a repeat of the same phenomenon.

If how the edge of the walk-to-your-front-door has become exposed over a 17 year period, is what you're talking about, all you need to do is add some topsoil alongside it, in order to re-conceal it There isn't another more efficient or more practical solution. But don't do this until the weather has warmed up and plants start actively growing. At that time, just add the topsoil and the grass will quickly grow through it, cover it and hold the soil in place.

As a long term maintenance strategy, it would be much better to add thin layers of topsoil over the turf every two or three years, to keep it flush with the walk, than it would be to add one thick layer every seventeen years.

It's the case that Google does not update it's Streetview pictures annually. The picture showing now could be several years old and, based on the state of the tree in the picture, I suspect it is. If the real conditions are such that the tree is producing too much shade for the grass to grow well, it's not going to be the plant that is capable of holding soil against erosion. In that case, you'd need to convert to some type of low growing, shade tolerant ground cover. If you have further questions about it, please post an accurate, updated picture that you've taken yourself.

It isn't erosion. The soil is not washing down hill. It's settling, and probably won't settle much more.

These pictures are so helpful to assess the overall state of your landscape. I envy you! This isn't the advice you came looking for, but I notice a deterioration of the the foundation shrubs. I liked the more natural and lower height of the shrubs 8 years ago. If you agree, that can be reprised. I find the mulched circle around the lawn tree to be too small now. I'd expand it to the size of the dripline. The two white pipes should be straightened, but even better camouflaged with plantings.

To regrade the lawn, do what golf courses do. Use coarse sand. The grass will grow up through it. To thicken the turf, use compost, overseed and water, all at the right times of course.

Thank you everyone for your comments.

I'm getting 2 different advises here: 1) add topsoil vs 2) use coarse sand.

I'm not sure which way to go?

A local garden center can advise you best, and sell you the right seed for where you live. Where I live you wouldn't use sand.

Golf courses use sand in very thin layers. You wouldn't want to add a 3" or 4" layer of sand to fix grade.