Does the use of an on-demand/realtime PC scan shorten SSD life?
yes it can be but not that much
However, it's better to use an AV with less disk IO activity with low pagefile usage
Some AVs cause too much disk activity that make HDD indicator in our PCs frequently light up. I heard avast and AVG cause it
I used avast myself and I can confirm it somehow. A solution to reduce it is to disable transient caching
avast is light in memory usage so after disable transient caching, I saw a lot less disk activity
1/ some AV also uses pagefile to reduce their memory usage seen in task manager so users may think they are light but actually not
for example, KIS 2017 have 2 processes in task manager: ~90Mb. However when we look at resource monitor/performance monitor/process explorer, they show at least 200Mb of working set (stored in physical RAM, not a problem) and 350-500Mb of Commit (might be partially stored in pagefile, not sure). Sorry, I disabled pagefile so I have no idea what value shows how much of the pagefile a program uses (maybe page fault delta - in process explorer?).
2/ Using to much of the pagefile can noticeably reduce PC performance especially with HDD and degrade it faster especially with very low physical RAM (1-2Gb)
3/ AVs with very slow scanning speed can also reduce HDD/SSD life span. This is what I think about Windows Defender/MSE. They causes too much disk activity at realtime protection
correct me if I'm wrong