Keep in mind that because few residential users max out capacity simultaneously the ISPs "overbook" capacity, and usually this works out because they have solid stats on average use and usually few people need the max capacity simultaneously.

Of course some ISPs are greedier than others and do it to the extreme where the uplink/downlink is regularly maxed out without giving anything near the promised bandwidth to a significant fraction of customers. The latter part should be disincentivized.

Force the ISPs to keep stats on peak load and how frequently their customers are unable to get advertised bandwidth, and if it's above some threshold it should be considered comparable to excess downtime, and then they should be forced to pay back the affected customers. The only way they can avoid losing money is by either changing their plans to make a realistic offer or by building out capacity.