That's true for dedicated circuits but no residential or even small business Internet plans are dedicated. You can put at least 200 customers on a 1 gigabit connection and they'll still get 100 Mbps+ speedtests even during peak usage times of the day.
My ISP seems to share your optimism, but I fear it must be grossly overprovisioned, or they have no intention of providing the agreed upon and advertised speeds.
Probably they're overselling at a much worse ratio than I described, or there are other problems in their infrastructure causing poor performance (likely both). It's not at all uncommon for ISPs to put even like 10,000 customers on a 10gbps line. 10 years ago that actually would have been fine, but bandwidth usage per customer has increased since then and naturally ISPs don't upgrade fast enough.
I've been on connections where the ISP (AT&T) tried this due to an ancient backhaul where it grossly over-committed on bandwidth. The neighbors would have kids home after school streaming three netflix 4k streams, and we would watch our speeds tank. Some bizarre QOS config seemed to prioritize their connection over ours. It ticked me off enough I almost started deauthing their devices because I couldn't load a simple web page.
On the other hand, we finally got FTTH and it's great. So nice to have good speeds.
