But that's true of almost every industry. Your bank advertises that you can withdraw your money at any time, but if everybody withdrew their money simultaneously there would be issues. A store advertises next-day shipping, but if everybody ordered simultaneously there would be issues. A house might have 100A power service, but if every house started burning 100A simultaneously there would be issues.
Put another way, what you're proposing is dramatically slower speeds for most users. ISPs are profitable, but they're not that profitable. Actual average usage on most lines is probably sub one percent. I just checked my line - my average use is about 300kB/s (for a total of about 800GB per month), or about 0.3% of my speed cap. I'm a heavy user, most people are probably below 0.1% of their speed cap.
If you want to know how much more expensive the connection would be if you expect to use the full bandwidth 24/7, just look at the cost you pay for transit in a datacenter. It is multiples of the cost home consumers pay at a per-bit level.
Most users are best served by being told the speeds they will usually get if they stay within average usage patterns, because that is what most people do.
Go to a grocery store in Florida before a hurricane and try to buy bread.