Ethernet is switched and under 20Mb/s is more than usable for everything unless you transfer movie files and wait, in which it would take like up a few minutes to watch a video. If you browse it's more than enough.
Re: the sibling's suggestion of wifi: If your cabling works at 100, I think the case is pretty clear for wired 100 vs wifi; consistent 100m is better than variable whatever you get. At 10m, not so clear, especially since cabling that's that old is also likely to be daisy chained, so you're looking at maybe a daisy chain of switches running at 10m. That said, cabling is often better than the spec it was tested to, and ethernet cable requirements are for long runs in dense conduit; it's worth trying 100M on old telephone wiring if it's already in the wall to see what you can get.
Let's not even talk about how miserable it would be to download modern games over this.
2. The idea in the comment is streaming at original bluray quality. Not transferring.
Also, I stream actual blu-ray rips over my network all the time, not just transferring them.
On a 120" display?
Would I want to use it to transfer large files around internally nowadays? No.
Works for everything else assuming you are the only user of that cable in that corner basement room? Absolutely.
Also insert appropriate "kids these days" joke. I guess it's like HD. Once you have it, you are not going back. Do I need a triple A game I just bought and want to play to download in 2 minutes vs 2 hours? No. But kids these days expect it I guess. No patience.
Or maybe they want to stream UHD content?