As I said above, your analysis builds in certain assumptions that may or may not be true (e.g., that a significant number of passengers redeeming miles at the last minute would pay for a C ticket). But yes, that is likely United's thinking.
On a transpac flight, you can upgrade from Y/B to C for 15k miles, or from M/H to C for 30k miles. You cannot, however, use any miles to upgrade from any fare lower than H. Thus, I don't see how it is reasonable for United to offer someone with an XY seat (essentially the lowest fare class, perhaps on par with G) the option of upgrading to C.
The practice to protect the C class seats is to ensure that United gets as much revenue as possible from a flight.
Generally, each mile is valued at 2c, and so 30k miles would be equivalent to $600. I believe that some folks in the past posted that United offer upgrades for cash. And the reason United would not accept miles as currency is simple - miles have an expiration date, cash does not (hopefully). If you cannot use you miles, the miles are of no value, and so United gets a "double take": the $600 in cash from those who are offered the option and are willing to pay, and your 30k miles that expire.
If you have to use your miles out of desperation, for example, for 25k on a domestic ticket that would have cost you only $300, UA "gains" another $300 on your miles.
Essentially, offering upgrades using miles at the last minute dilute the "value" of the C seats. Those traveling on business may purchase Y/B instead and ask for their "15k" upgrade at the last minute -- and United can lose big. Still, if the "last minute upgrade program" were to be implemented, XY would be at the dead end of a queue, as it would be easy to find 70 or so people who have purchased Y, B, M, H, Q, V fares, let alone those in W, S and T.