That's not really how it happened... Yes, there were ultimately two companies involved, but the dual contracts were very much government planned and funded. With, largely, the explicit goal of replacing the privately built elevated lines.
Never mind that it's the IND system that really gives the subway anything like the scale it has now, or that 2nd Ave was the first major opening since the IND.
That said, I'm with you on the general sentiment. As a matter of principle everything about the REM process seems a bad idea, but otoh if (quasi) private assumption of the planning can get things done as fast as they were in Montreal... The issues it will probably cause seem far prefereable to dealing with the outcomes of not doing anything meaningful for decades at a time.