But why /65 is such a big deal? You can divide it in 32 different /70 subnets, one per company. That still leave you with 2^58 IPs by company. You can still even use MAC addresses in your IPs.
SLAAC on ethernet requires a /64. A provider assigning you anything but at least a /48 is pure incompetence, if it's explicitly for 20 companies, they really should be giving out a /43.
Also, no, it's not really just a minor problem. Reconfiguring your one client device might be trivial, but that's really not the question.
The question is what you do with a setup with a few dozen devices/machines/routers/whatever.
First of all, even if you do a new setup, it's idiotic that you'd have to consider the idiosyncrasies of your ISP when designing your network, instead of choosing whatever setup fits your internal requirements best. That you possibly cannot buy a certain printer because it doesn't support DHCPv6, or whatever.
But it's a lot worse when you think about switching ISPs with an existing setup: Having to completely rebuild your network because of your ISP's idiocy is ... well, idiotic. Suppose you have a bunch of routers that do SLAAC for end devices and obtain prefixes via DHCP-PD from the uplink router. Really, all that should be technically required in such a setup to switch from one provider to another would be to unplug the old line, plug in the new line, everything should renumber automatically, while keeping the exact same network structure. Good luck reconfiguring all of that to work with a /65 ... and when you have done that, tell me again that it's a minor problem.
You don't need to set up your clients with IPv6 more than you need with IPv4. Your router will do DHCP in just the same way. To me this is only a minor inconvenience.
You might as well be saying that electrical installation is trivial, as it's just a plug you need to plug into the wall.
All the stuff that I've been talking about is what is required so that in the end, you can just plug in your laptop and have IPv6 connectivity without any manual intervention on your part. Yeah, that's how it is supposed to be. But that depends, among other things, on the ISP providing a reasonably sized prefix.
Also, there is more to networks than "the router" and "clients". You can have more than one router, you can have servers, you can have routers with multiple interfaces that are isolated from one another ... not everything is "my home DSL router with built-in WiFi access point, used by one smartphone, one tablet, and one laptop".
And I think you might be confusing SLAAC with DHCP? Those are two completely distinct mechanisms, and you don't need DHCP in IPv6 networks at all, in particular not for client systems.
