This shouldn't have to be said to someone in your position but you are demonstrating a lack of knowledge about the united operation. This was/is the same lack of understanding that lead to the downfall of our management team. We aren't an airline with one hub. We don't fly for delta. This isn't a delta operation, doesn't work like a delta operation, and you aren't dealing with delta managers. The sooner you realize you don't understand how United operates the sooner you can start learning.
We tried checking in years ago and it was a nightmare. Both management and our union gave it up because it just didn't work. Saying every other airline does it means nothing. Does every other airline have crew rooms a mile walk away from their normal gates? Does every airline operate up to 2000 flights a day? No! So stop using the same broken, ill conceived excuses that our management tried. If you had any idea what you were talking about on this it would be a no brainer.