Let’s be clear. The Democrats have no mandate to pursue impeachment. They have a majority in the House due to the election of forty or so moderates who won in districts Trump took in 2016. Most of those winners made pledges of one sort or another to not pursue impeachment. Without a smoking gun or some new facts that convince a large swath of swing voters and Republicans, a push to impeachment will simply hand the House back to the Republicans in 2020.
The release of the Mueller report and his subsequent testimony convinced most moderates the collusion narrative was false. After two years of hype, it all turned out to be a big dud. To continue to push for impeachment now just vindicates Trump in the eyes of a majority. The Democrats who still demand impeachment without plausible high crimes and misdemeanors are seen as simply trying to overturn the election of 2016. As for the barrage of subpoenas, they will eventually all be quashed in the Supreme Court. The Congress and the Executive are independent co-equal branches. The Supreme Court ruled in Nixon v US the Executive does have broad executive privilege but it is not unlimited and is subordinate to a court request for specific materials needed for judicial proceedings. But there is less precedent in support of Congressional subpoenas, much less their overt political use in “subpoena cannon” mode. Recall in the Obama Administration, Attorney General Eric Holder successfully defied Congressional subpoenas related to Operation Fast and Furious and argued Congress had no authority to compel his compliance. Unless there is an actual impeachment proceeding underway, none of the current House subpoenas will ultimately prevail.
However, in the end, Nancy will have to give in and allow a vote on impeachment as demanded by the Progressive Left wing of her party. They won’t shut up without it. Meanwhile Trump wants impeachment and then a Senate trial all cresting in his acquittal in the middle of 2020. His strategists believe those moderate districts will then be more likely to swing back and that Trump will stand a better chance of winning the popular vote and a huge electoral college victory.