To be clear, the OLC's policy was the automatic disqualifier for bringing charges against the POTUS anyway and Constitutionally well founded. That is the job of Congress in this circumstance. Whether Trump would have been indicted? Who knows honestly. To be fair with this, Trump is inferred in a lot of indictments, including Manafort and others, without direct naming. However, we'll probably never know if dude was guilty or not. Simply put, any Impeachment resolutions passed by the House will be simply be going through motions in the Senate as it would be probably DOA anyway. It's such a broken system.
Even though the DOJ can't criminally indict a sitting president, the Democrats were hoping that the Mueller investigation would provide enough evidence to pursue impeachment, which, as you pointed out, is a political process. Mueller's finding that there was "insufficient evidence" to show a criminal conspiracy between Trump's campaign and the Russians, really undermined that effort. Democrats were really disappointed by this.
Nonetheless, the Dems want to get rid of Trump. If they can't find grounds to impeach him, or censure him, they still want to create enough political theater and enough ruckus in the media to try to vote him out in the next election.
Of course, the Dems and liberal-leaning media are fully entitled to do this, but let's not pretend that this hasn't been politically motivated from the outset. And all this talk about "doing what's right to preserve the integrity of our Republic for future generations" comes off as shallow, faux outrage that insults everyone's intelligence.