My conclusion was it was not worth the time to go through old files image by image.
I try not to make the problem worse by rating every photo I take on a scale of 1-5. I discard my 1 and 2 rated files, keep my 3 rated files just in case, and only edit a subset of my 4 and 5 rate photos. For example, I might have 4 frames that are very similar rated 4, but I just pick the one I want to edit (using a color tag) and don't dwell on whether it is the perfect photo since I keep them all anyway. I do all the front end rating and other tasks in Photo Mechanic because it is so fast and does not need to create new previews. It uses the embedded JPEG in the RAW file. I only import to LR the images I plan to edit.
This whole rating system emphasizes touching images as few times as possible, and creating a buffer in case I miss something. I would rather be fast than perfect. Today I downloaded 250 images from a shoot yesterday morning, rated all the images, identified Selects and imported them to Lightroom, did a quick edit, exported web JPEG files, and sent the 30 JPEG files to the client. The whole process took about 90 minutes.
My long term storage is external hard drives - three of them. I use 8TB external drives with a primary onsite, a backup onsite, and I swap out the onsite backup with an offsite copy every month or two. I use a program called Beyond Compare to synch my backup drives and update any changes. Blue Ray disks, DVD's, and CD's are short term storage and are not safe for long term storage. Images will deteriorate on BR disks.
I try to keyword all files in Photo Mechanic and add captions and titles. I probably get that done on 95% of my images each year - usually within a day or two of shooting. Every now and then I get backed up and only have preliminary keywords, titles, and descriptions on some images.
When it comes to old files, I have not gone back to hard drives and updated rating and keywords. It would be a good idea, but I have 100,000 old images that lack that data and 100,000 that have keywords and ratings, and 100,000 that just have ratings. It's just a matter of priorities.
I use cloud storage for my best photos in addition to my backup drive system. I keep a copy of the RAW file and a finished edit for most files. I use iCloud and Amazon Prime Photos for free storage of about 1500 files. I don't expect to need these photos, but I have them just in case.
When it comes to old images, you have to balance the benefit of freeing up space with the time and effort it will take to go through the files, figure out whether it has value, and to keep or delete the image. If you were able to free up 20% of your backup drive space and you have 2 TB of images, you're still talking about an insignificant storage cost benefit. 400 GB is a rounding error when it comes to storage drives and the incremental storage is probably worth less than $25 per drive. With 3 drives that's worth $75 - about an hour of your time or less. Just keep them.