Deteriorate. It may imply someone’s health is getting worse; it may also suggest that a business or other endeavor is failing. ‘Downhill’ is used,  not as a vertical metric but in the sense of going from the bottom, from good to bad. (Compare to Go from bad to worse or Out of the frying pan into the fire.)

“That was a nice house when I was a kid, but it sure has gone downhill since.”