Regarding - 'folks living in the flood plain', this is a relatively minor issue. Bonneville was closed around 1938; european settlers had been living on the flood plain since the 1850's or so. In fact, there were more people living along lower Columbia pre-WWII than there are now. Spring high water was just a fact of life.

Beyond the dams, in the locales where the greatest capital investment has been made (like PDX) the river is constrained by dikes that will hold back a flood crest at 32' or so.

Regarding 'will the dams come out'. Sooner or later they will no longer be. All that sediment is being deposited behind the dams, filling their pools in.