There are some reasons, which is why you do see IPv6 use increasing. IPv4 exhaustion means that almost all mobile (and in some countries landline) internet connections have to access the IPv4 internet through Carrier Grade NAT. ISPs have to buy the equipment to operate these and pay for their maintenance, and they have to do so in proportion to how much traffic is stuck on the IPv4 internet. At a certain point making the necessary investments to send more traffic over IPv6 end-to-end becomes a better bet than continuing to maintain a growing CGNAT stack.

The tough part is that while ISPs can largely control whether their mobile and residential users have IPv6 available they can’t really do so for their business users, let alone arbitrary website operators they have no relationship with. So the reality is that everyone is going to have to maintain both 4to6 and 6to4 basically forever. But as it becomes less common it’ll no longer need to be especially fast or efficient and the costs to operate it will come down.