>Block producers (miners) must find buyers for the blocks they produce, if they don't find buyers, they go bankrupt.

In a buyers market. With the small block size, bitcoin is a seller's (miner's) market and refusing to upgrade preserves their market power - hence the tragedy of the commons.

>You've got it backwards, changes to Bitcoin need to be accepted by block consumers (node operators).

The number of bitcoin nodes has been dropping for years. The rational actors who are incentivized to support the network are making the decisions right now by choosing which forks to support.