I think we're coming closer to agreement, though I'd stipulate this liberalizing of doctrine is not actually "mainline". Yet.
Certainly among the leadership it is becoming so, but if it were among congregants their numbers wouldn't be descreasing at the pace they are. Of course at the end once all those that take scripture seriously conclude their exodus, the majority of those remaining will fit your stated opinion and, infact, be the new "mainline".
If I am not mistaken, "mainline Protestant" refers to Anglican, Episcopalian, Methodist, Congregationalist, Lutheran and maybe Presbyterian. They all for the most part have gone over to the dark side, and the transition has been driven from the top down, while the flock fled. It is mainline in the sense that the denominations are labeled "mainline" to distinguish from "fundamentalist" and the leadership approved of the liberalization. I agree that the issue is not settled yet, particularly where the leadership has to listen to pastors outside of Britain and the US, who tend to respect tradition better.