The problem with private address ranges is that everyone thinks they're available. In a large enough enterprise you're bound to have conflicts. They usually pop up at the most inconvenient time and suddenly you're cosplaying ARIN in your IT department.

> everything fit's nicely in the 10.0.0.0/8 range

Except during a merger/acquisition and both companies have 10.0.0.0/24 in their OSPF or IS-IS topology.

> everything fit's nicely in the 10.0.0.0/8 range

Except for when it doesn't.

If you just use that space as a flat range, it is almost certainly more than enough. But if you split it up in multiple levels of subnets, you can run into difficulties balancing having enough subnets and having enough space in each subnet.

We burned thru pretty much all of our public /8, RFC1918, and have begun digging into RFC6589 (a /10 I didn’t even know existed prior to job). Still shocks me. Hardly an expert in the space, but I think the issue comes from subnetting to distribute ranges to teams that need a consistent IP address space for some project or another. Lots of inefficiency & hoarding over time. We’ve had legitimate outages and impending platform death staved off by last minute horse-trading & spooky technical work due to such things. IPV6 has always been a distant aspiration.

Grow large enough and you hit the limit pretty fast. NAT complicates things.

you haven't had to set up intercompany vpns I see

Unless you get to big. Or you merge with another company and have to combine your internal networks and oops, all the subnets are overlapping. Or you need to serve mobile clients who get better connectivity over v6.

if both you and companies you have site to site vpn with have IPv6 there is no IP conflict or NAT to worry about.... and that's about end of the advantages

one poorly made decision and oops you're out of 10/8 addresses

if you've never run in to this, then sorry, you've not been in an enterprise, you're in a mom 'n pop shop cosplaying as enterprise.