From another perspective, Pelosi is keeping the Inquiry from being openly voted on to protect her members that are up for reelection in 2020. She doesn't want those who barely beat their Republican opponents in red-leaning districts to be forced to vote for something their constituents mostly know is a lie. If those districts helped elect Trump, they can't rely on TDS to give them the win in 2020. Voting for a bogus inquiry based on bogus (not unlawful) scandals will backfire on them.
Also, once the inquiry is official, assuming the vote goes their way, the process becomes less one-sided. Republican members can then call their own witnesses, cross-examine the Dem's witnesses, and attend all the hearings. Without the vote, this is nothing more than another partisan, political witch hunt.
More voters will turn out in 2020 than they did in 2018, because it's a presidential election year. Midterms usually have a much lower turn out. That fact alone means the Dems are at great risk of losing the House majority again after just 2 years in the Speaker's seat. especially since the Dems haven't had anyone in the primary field that can excite their party voters and motivate them.