You allude to 100mbit satisfying a house of 10 so I'm going to latch on to that.
Avg web page size=3MB/24mbit https://speedcurve.com/blog/web-performance-page-bloat/
Start render might be about half the page, say 10mbit to be round. A 100mbit connection might take 0.1s to start rendering the page, while 1gbit takes .01s. 1000 page loads avg per day, 30.5 days a month, this is ~45 minutes.
Streaming audio/video quality is proportional to bandwidth, and 100mbit can incur performance hits, depending. No you don't need a gigabit, but just factor in the rare but not completely inconsequential occurrences.
Unknown native app bandwidth changes. It is smaller but probably not inconsequential. I'll skip it to be generous.
Your large downloads easily save 5-10 minutes. How many large downloads a month? Maybe 2? Well that conveniently gets us to 1 hour a month. That is each person in your house. How much does your household get paid? How do you value your time? Family time? Free time? Are you sitting there very every web page load (yes)? Are you sitting there for every large download (no)? Is this very hand-wavy (yes)? How much time is otherwise occupied by work, kids, life?
Where are you spending your very few dollars difference to get the value an extra hour a month per household member would get you? There is some function of salary and valuation of free time where gigabit makes sense.