It's all about supply and demand.
When times are good and the industry is healthy, jobs are relatively plentiful and airlines' bars are set at a correspondingly low level. During recessions, when recruitment is at a virtual standstill, airlines offering the few available jobs can set pretty much whatever terms and conditions they like, knowing they will still get more suitable CVs than they know what to do with. For the more enterprising airlines (or unscrupulous, if you prefer) those terms and conditions include SSTR or pay to fly schemes. These schemes only exist because people are prepared to pay. People are only prepared to pay because they are desperate, and they are desperate largely because there are very few jobs available.
SSTRs have been around for a while, but are always more prominent during recessions. I am thinking of British Midland in the early 1990s, but I'm sure someone can find an earlier example. I doubt they will ever disappear completely, regardless of the health of the industry, as there will always be those who can't find a job any other way and have access to the cash to pay for a type rating or right hand seat. When the economy eventually improves, however, more jobs will be available and fewer people will feel the need to spend another £20k plus on top of their CPL/IR for the privilege of flying a 737. SSTR and P2F schemes will hopefully sink back to a low level – until the next recession.