Tester was declared the winner of his race within the hour so there will be no more losses. I am chalking FL up as a loss as I just don't see how he can bridge a 30,000 vote gap. I can't figure what's going on with AZ as only 75% of the vote is in and only roughly 16,000 votes separate the two of them. If Sinema pulls it out, they Republicans will end up only +2 which is a small gain.
I agree that the Democrats desperately need to start grooming a new generation of leaders. Pelosi-Hoyer-Clyburn are the same three who led the caucus eight years ago. One good thing about last night is that the Democrats can finally start building a bench again after it was obliterated downballot during the Obama years. By 2016, all the party's eggs were in Hillary Clinton's basket and we saw how that worked out. Although the nation and world would certainly have been better off if she won, I am sure there is an alternate universe somewhere where she did win and last night was absolutely brutal for the Democrats. Think at least 20 House seats lost, no pickups in the Senate and Tester, Menendez, Manchin, all going down giving the Republicans 59 Senate seats and the ability to dream of a Republican President with supermajorities in both house of Congress come January of 2021. Back to this universe, the Democrats' weak presidential bench is the biggest drag on hopes that the party takes the Oval Office back in 2020.
As for the Democrats messaging problem, I think it goes to the fact that many Democrats and liberals come from a legal or public affairs background and tend to try to appeal to their heads and sell things rationally in terms of lingo and legalese. On the other hand, many Republicans come from a business, marketing and, by extension, sales background which focuses on appealing to peoples guts and making the people think they want what they are selling.
And they did pick up seven governorships and can now claim that a majority of the American people now live in states that have Democratic governors.
Winning the House is not insignificant for other reasons in addition to the ones you state. In addition to being a check on Trump, Obamacare is safe from repeal for another two years (and,the longer it stays around and people come to rely upon it, the harder it becomes to repeal) and Trump's requests for border wall funding to the extent that he wants will most likely be dead on arrival in the House.