While true in general, the cost of unoversubscribed wholesale bandwidth is rapidly approaching the cost of oversubscribed retail bandwidth.
Wholesale bandwidth is 5 cents per Mbps or less at scale. Not that the ISP pays for all bandwidth, peering and CDN traffic is free.
In other words, even if you max out a 1 Gbps line 24/7, you are only costing the ISP $50 per month.
Add to that that most people are unable to consume even 10 Mbps on average and bandwidth costs become less and less of an issue. An average subscriber will use 2-4 Mbps of bandwidth during primetime, and primetime is all you care about as an ISP.
Nothing in this means that offering bandwidth on demand is impossible or a threat to the ISP business model.
It’s just that the overhead of offering an on-demand product to consumers isn’t worth the time and effort, as you’d be making peanuts while having to deal with a lot of grief.
Bandwidth on demand is a thing for businesses and operators. A good example of this is Megaport, who offers virtual cross connects and transport. Price start from $100 per month plus a varying fee per Mbps, from a few cents to a few dollars per month.