I have no idea why Democrats have agreed to Trump's battlefield conditions.
They've been doing this since the 1990s, so no reason they'd stop now. The problem is that they haven't figured out how to either raise the standards of debate or beat the Republicans at their own game. Meanwhile, it's 100% clear that the American electorate will not hold the Republicans accountable, and the Electoral College has failed to serve its actual purpose, which is preventing an unqualified demagogue from ascending to the presidency. As I mentioned elsewhere, we as a country get the government we deserve, and our failure to think beyond short-term self-interest, bigotry, and party affiliation means we willingly elected the most obviously unqualified person ever.
But the bigger problem we're facing is not Trump per se. Instead, it's the fact that we've allowed Congress to completely abdicate its responsibilities and governance for the last couple decades. We've grown to expect Congress to do nothing, and it's totally okay for them to sit idly by while the executive does everything by fiat simply because they want to or feel like they have to because of McConnell's refusal to put anything to a vote. About 800,000 people in Kentucky have thus been able to stymie the wishes of every other voter in the country. Things kept getting done in some way, shape, or form, though, so no one cared.
And no one will continue to care until it affects them personally. All this talk about corruption, about damaging the Constitution, the office of the president, whatever, is far too abstract. Until Trump's greed and stupidity hurt individuals or people they know, he can do literally whatever he wants. Fewer people support impeachment now, after the hearings, than did a month ago.
We decided that we'd rather have elected officials who pander to us like someone running for class president in their high school than someone to actually lead.