Corrupt files appearing when I open Lightroom

Is the thought that somewhere along the way these images were corrupted

YES

and they're just decaying over time

Probably not.

Do you have backup copies of these images?

Thinking about this a bit further, it is still possible the preview database is corrupt and that the originals might be OK. The description of the images coming up more and more corrupt every time Classic is open is suspect as the icon on all the corrupt previews is still that of embedded preview. These previews are based on jpegs that were initially on import copied from the raw files. If you don't actually take these into develop and back, the previews never get updated and the actual raw files are not read again from the external hard disk. So something for the OP to try out is the following. On one of those corrupt looking images, take it into develop. Does the corruption stay or disappear? When you go back in Library (move a few sliders back and forth in Develop), do you see the preview fix itself after a few seconds? This would indicate the previews Lightroom maintains are somehow being corrupted which does happen.

Again, I would also mainly suspect the external hard drive but there is a (small) chance that it is the previews on the internal disk that are problematic. 

Also, I would run Disk Utility First Aid asap on both your internal and external disk (just type disk utility in the looking glass right hand top of meny bar). Perhaps do this before anything else.

P.S. Thinking about this further: how is your external disk formatted? Hopefully hfs+(Mac OS extended) or APFS? If it is NTFS or a form of FAT that is not the best choice and I would move all the data to another disk that is formatted with an Apple format. Many exernal hard disks come formatted using windows compatible formats which are not well supported on Mac OS X (to be able to write to NTFS you even have to install a third-party utility that breaks almost every system update) and before use on a Mac you should reformat them. Don't do that now but only after you moved the data and know the data is safe and complete on another disk that was correctly formatted first. This is a secondary problem to check out after you have done some recovery. Also, I might be telling you nothing new and your external might be correctly formatted. It is just really common for people to never do this and just plug in a new external disk and start using it without first reformatting it. 

@Jao vdL If you look at the top left corner of the thumbnails you'll see the badge that indicates that  it is based on the embedded JPEG preview rather than a preview created by LrC.  Therefore, it's almost certain to be the case that the files are indeed corrupt.