@NF16 There are a few points here I can add something to.
About studios remastering old movies, it's just not going to be cost effective for them in the current market. I mentioned before that the cost of re-digitising old movies, that were originally shot on film, with modern technology would be prohibitively high, considering the tiny amount they would sell. I would also guess that getting a pro to remaster old movies wouldn't be very cost effective either, because there really isn't a market for individual titles anymore. Consumers are used to an "all you can eat" model like Netflix or Spotify, so expecting consumers to pay the price of a subscription for just one old movie doesn't make any business sense these days. It makes far better sense for the studios to put all their old content, in whatever digital format they have, either on their own subscription site or one one of the subscription streaming platforms. That way they can still make a few cents from their old titles without making any effort.
I have seen a few legal sites with 720p and 1080p versions of older movies that would have been shot in SD. One user here shared a load of Pacific Sun movies from one of those sites, and they were all just 480i DVDs ripped at the wrong resolution, and often the wrong aspect ratio and not deinterlaced. Absolute garbage and not worth anyone's money. Imagine a musician releasing a remastered lossless version of their classic 1970s album, and finding out that it was just an old 8-track copied to a PC in 128kbps and then converted to 24-bit FLAC.
I mentioned the Falcon series further up, it's funny you mention it too, because I'm still not over how bad they are, and that Falcon thought they were good enough to release. I actually wouldn't consider them studio remasters, because they haven't used the original recordings. They used DVD rips, and not even high quality rips, because they weren't deinterlaced correctly, so the combing artefacts are now burned into the image forever. How exactly they were "remastered" isn't at all clear, because they just look like they were very crudely processed with a load of Instagram style filters. It's 100% an amateur job, because I don't know any professional who would do that.
You're absolutely right about the quality of the source video for remastering or upscaling. There is a golden rule: in = out, and that is 100% true. You don't need to be a professional, there are some forums I am on where amateurs are restoring old videos to an amazing quality, but they have been doing it for years, and sharing their failed attempts for advice. Anyone remastering older content should want the best quality, as close to the source as possible, ideally the original tapes (if they have the equipment to play them), or at least a good quality transfer. I don't know if most porn studios kept tapes or reused them, because some formats would have been pretty expensive so if they wanted to save money they would have reused the tapes once the content was transferred. There are a million variables that would affect the quality of that transfer, so it may be really good or it may be so bad that a professional with any integrity wouldn't touch it. In a lot of cases really poor transfers are still being used to produce DVDs and for web content, because that is all that exists. Take one of these poor transfers, burn it to a DVD, rip it to a compressed format, upload it to a streaming site where it's encoded again, download it and pass it through AI software to turn it into "HD". You might expect the results to be quite poor, but did you think they would be this terrifying?
Try not to have nightmares!