Good post Larry, explaining the maths. My take-away is that data retention is somewhat higher for HDDs but not by a great deal.
The underlying failure rate figures are the key though, and they might be somewhat rubbery. The recovery rate figures are likely to be quite accurate with the only caveat being that without expensive specialist recovery services (often not affordable to home and small office users) that rate would be lower (60% ?).
Considering the SSDs are getting better all the time, and if sticking to certain major brands, and the different usage patterns in client PCs compared to Backblaze's server usage, the failure rates might actually be lower. My own experience suggests that the SSD failure rates are lower than 1.5% and entry-level HDDs made in the last 10 years have a higher failure rate than 3.5%.
Assuming Backblaze's failure rates though, with the high costs of specialist recovery I think it tips in favour of SSDs (for home and small office users) in regards to data retention.
Regardless, I agree with "regular backups". This has always been and always will be the advice for any storage medium, another reason that the relatively minor differences in data retention between HDDs and SSDs is not significant.