As I understand it from your earlier post, the aircraft was an A380 and then got changed to an A330. That is a HUGE drop in capacity in Business Class seating.
I'm thinking, depending on the configuration, the seat you had selected (by your partner I assume?) may not exist in the A330 config (you mentioned your partner kept his seat allocation). Now yeah, if they needed to "find" 30 odd seats (depending on booked load) then yes, a low/non status CR booking (and to QF, a pax going solo) would be a prime candidate to move via MEL for better or worse.
In this scenario it's almost a certainty that there were no extra seats sold and even if a Platinum (or P1 or CL) might "want" to get on that flight that's irrelevant. The aircraft swap would have resulted in zero capacity for anyone (hence moving pax like you), but even so high status pax can request things, but if there are no seats there are no seats. They won't turf off a ticketed passenger because someone else wants to get on (specially this far in advance, if in an irrops situation and a VVIP is accorded some sort of special treatment for whatever reason that's potentially possible, but still unlikely imo. Anyway not an issue here).
So, I'm going to suggest you were bumped off because of the reduction of capacity resulting from the aircraft swap - and not for any other reason involving other passengers or to get more revenue from selling "your" seat.
Having said that, no it's not cool that they changed the aircraft for the flight since it's affected pax (almost definitely not just you alone). There are obviously reasons why this was done, but it shouldn't be relevant to passengers who had seats and were forced to move (like you).
my two cents.