The unspoken reason for the recent SP decline is the impending...

The unspoken reason for the recent SP decline is the impending availability of rapid point of care (POC) antigen test for COVID-19.

https://biotechdispatch.com.au/news/atomo-gains-rights-to-rapid-covid-19-diagnostic
https://www.ellumehealth.com/2020/09/10/ellume-together-with-partner-qiagen-announce-high-performance-test-for-active-covid-19-infection-with-results-in-less-than-15-minutes/

The rapid antigen test gives results in 15 minutes. Only 8 samples can be run concurrently. In the symptomatic patient, within the first week of illness when viral load is high, sensitivity is up to 90%, specificity is up to 100%. Accuracy is probably inadequate for later stage illness or asymptomatic individuals.

I think a lot of people have been worried that these rapid antigen tests could replace PCR.

However I don't think that is the case

https://www.tga.gov.au/covid-19-testing-australia-information-health-professionals

From the TGA website:
"Rapid antigen tests for viral protein

Rapid antigen tests intended for use at the point-of-care detect the presence of viral protein from SARS-CoV-2 and may be used in the diagnosis of a SARS-CoV-2 infection in a symptomatic patient. COVID-19 antigen tests are generally intended for use with nasopharyngeal, throat or nasal swabs and testing should be performed by health professionals in accordance with the manufacturer's instructions for use.

While rapid antigen tests can provide a result within 15-30 minutes, they are generally considered to be less sensitive than a PCR test which is still currently the gold-standard in SARS-CoV-2 diagnosis.

Rapid antigen tests are best performed within the early stages of acute infection, when viral load is at its highest levels (i.e. within the first 5-7 days from symptom onset), after which antigen levels may drop significantly. In conjunction with clinical findings, a positive result is generally considered to be accurate, however further testing by PCR may be required in some cases. A negative result in a symptomatic patient, would require further confirmatory testing via PCR testing. The clinical utility of rapid antigen tests in screening asymptomatic persons has not been established."

I personally can't see POC antigen testing to be accurate enough to replace PCR.