Alright, I'll engage. The -135 has some advantages over the -10: they have a datalink, they've got their GATM upgrade done, and there are 6 times as many, so they've got a lot of booms in the air. Otherwise, here is why the KC-10 remains the best tanker in the world and why its crews are so damn proud of it:
1. We're the only tanker (existing or planned) that can support a real long-range fighter movement capability. Considering the future Pacific shift, this is vital. If we're ever going to try running an air war out of Guam (IAW the RAND analysis from a few years back) we'll need some pretty gigantic offloads to move a 2-4 ship of large air superiority fighters 3 hrs west and back. Boom sequencing won't be an issue. Ref 1986/2011 Libya ops.
2. Global strike and airdrop require even greater offloads. KC-10s actively train for and execute these missions; moving large aircraft across the world nonstop will be greatly degraded/impossible without us.
3. Every KC-10 can refuel any allied jet on every mission. Any future conflict will be joint AF/USN/allies and real-time flexibility will be required. This is why the same capes are being built into the KC-46.
4. Every KC-10 Aircraft Commander is a fully qualified/current receiver and formation pilot, and we're good at it. Not only does this give us a lot of operational options (again, on every mission we fly), it makes us better at tanking because we know how much it sucks to be snap-rolled into the sun or weather. Fun party trick: we can boom-check our own formation members to ensure the systems work before the users show.
5. We're cheap to operate: $21K per hour, not including fuel (add another $9K for that). I doubt there's any jet in the inventory that can move as much payload for so little cash. There's a reason FedEx still uses this airframe. And as far as the avionics upgrade cost, it's less than the cost of one KC-46. Pretty good value, I'd say.
6. Yes we can haul a shitload of cargo, almost as much a C-17, just not the oversized stuff like MRAPs. But if you have 160K lbs of gold bricks that need to be in Japan tomorrow, we can get it there quicker and cheaper! Also, there seems to be a perpetual myth that since we carry a lot of cargo, we must be inferior tanker pilots. This myth is dumb. Flying cargo around into non-tactical environments is easy it doesn't detract from other skills.
7. What Boom Control Unit issues are you talking about?