Last thing an ISP wants is for all it's users to move to a slower internet connection because a good QoS lets them avoid the lag/stutter of VOIP, video calls, gaming, etc.

Consumers shop for routers and ISPs based on things besides QoS.

Because you need to specify the bandwidth, which the router maker cannot know?

Not really, fq_codel + bbr can work based on RTT changes alone.