Democrats who preemptively declare impeachment off the table are mistakenly (or intentionally) conflating one possible end result of the impeachment process for the process itself. The Republican members of Congress who voted to open an impeachment inquiry into Nixon’s conduct didn’t necessarily want it to end in his removal from office; even up until his resignation it was an open question whether there were enough votes in the Senate to remove him. They were trying to get at the truth about the administration’s actions, and using impeachment to gather evidence. (They didn’t even limit themselves to Watergate. The committee eventually also voted on whether to impeach Nixon for the illegal bombing of Cambodia and for failure to pay taxes.)
All of this.