This is hardly scientific, but I've just been through transferring all the programs on my 40+ year old tapes to digital files (via a PET2SD) and the cassettes that caused me most problems were all commercially produced tapes.
Colin.
If magnetic tape can hold onto its dipoles for more than 30 years it is actually doing pretty well. Where the task is more demanding, say video tape, where you can see the dropouts visually, many VHS tapes barely made it past 15 to 20 years before the data faded away. But in some cases, depending on the quality of the media, data and images from tape over 40 years old has been recovered.
One interesting video was a lost David Bowie recording not seen since 1973, the BBC wiped the original tape, but luckily there was a copy. The video quality looks very good, though it was subject to some repairs and the original tape stock was probably very expensive & good, this was the story:
"After the broadcast the BBC repurposed the tape so the performance was presumed lost until 2011, when John Henshall, a cameraman on the show, revealed that he had 2-inch tape copy of broadcast quality".
Perhaps one possible explanation why the original commercial tapes gave you more problems, they may be physically older than copies of programs, made later, by the users with the PET & Datasettes, but it is hard to know for sure, or maybe the copies were made to better quality magnetic media that has lasted longer.