Again, the only difference is that on-disc content saves me bandwidth. I don't really care where the expansion is, I choose whether to unlock it or not, see the example of serial codes of the olden days above. If I want access to that additional content, its source doesn't concern me, only its quality does. As long as it's an actual optional expansion and not a clearly missing part of the game, I don't see the issue. Again, you don't own content, you only own a piece of plastic. Another example could be Windows installation discs - they have the complete system on them, but only the modules relevant to your edition of the serial are installed. Accessing the rest is as trivial as changing one config file and getting a serial for a different version, at least as far as I know.
Technically, you are completely right. I don't think the problem with on-disc DLC is a rational issue. Instead, it's an emotional issue that has to do with people paying a lot of hard-earned money and then finding out the experience they got wasn't as complete as they'd like it to be.
Consider a game being released in the N64/PlayStation or PS2/GC/Xbox generation. It would probably be loaded with free neat extras, like extra costumes, bonus stages, etc. At the same time, PC games of that era received updates that made the game run better, added some extra options, etc.
In modern games, the neat content that is created may very well be sold as DLC instead. That is, the content is done, but the developer chooses to sell it instead of giving it away for free. Considering 60/70 dollars/euros is a lot of money, I understand why this annoys people. Other people will even get annoyed at small DLC being sold when it's released after the game has been released, simply because they're used to getting it for free through free content updates.
Some of these views come down to entitlement. At the same time, there is a tendency of publishers to more and more nickel and dime customers that already pay a big sum of money for a game. I think there will always be extremes of the spectrum (The Witcher 3 and Left 4 Dead 2 on one side, Batman: Arkham Knight and Dead Space 3 on the other). Everyone should simply decide which DLC they're okay with, and support that content with their wallet while "boycotting" the other stuff.
To conclude, I think you're right: having DLC already on the disc can only be good for customers, since it saves downloading extra stuff. It's not this practice that gamers take issue with; instead, it's a sign of greedy business practises they don't agree with.