Your question goes to the heart of the problem we face in a digital age. I've seen the internet and computing mature over thirty years and because it's all about living in the moment technology and content is simply discarded and forgotten. One of my favourite Macs is the first iMac, but its chief value now is as a novelty item - anyone remember the classic macOS? Any game made for older operating systems now cannot be played, and so on. Even in the professional arts the same problem exists: EMI wanted to remix the original recordings of Pink Floyd's Dark Side of the Moon album for a special anniversary release, but to their horror realised that they did not have the hardware at Abbey Road to play a 2 inch audio tape. They had to import it from a branch office in Asia, I believe, in order to convert it to digital format - and learned their lesson about matching hardware and software.
In terms of preserving photos, your metadata will be secure and transfers with the photo. In the short term, future versions of Photos or its successors will be backwards compatible and be able to read the various current formats of image files, in a century though who could say?
But librarians are already ahead of us. Being a retired librarian I was long ago aware of the efforts of the major collections to help preserve digital photos. The American Library Association for example has this page which is a good starting point if you're interested in the topic, and here in Australia the National Library has similar advice on this page. The British Library has this program to assist people preserve their photos, and so on.
Mind you, our forebears from a century ago had the same problem with printed photos. Which family does not have some old snaps with a hastily scrawled note about when, where, and of whom the photo was taken? If you're lucky you can read the antique handwriting. Nothing changes does it!
