The biggest thing I don't get it is that virtually everyone in the line of succession would sign any bill Republicans put in front of him. They don't need Trump unless the conspiracy is so huge that the entire line of succession is guilty. And even then, it will take so much time for the conspiracy to unravel (remember, Nixon had a probably much smaller scandal and he still took three years) that it doesn't matter. Trump goes down? Pence will sign the same bills. Pence goes down? Ryan will sign the same bills. Hatch will sign them. Tillerson will sign them. They don't need Trump.
I think it's two things. Republicans don't want the drama and the damage done to their party if impeachment processes would become the major focus in Washington. Democrats are hesitant to demand impeachment because they don't have power and it's an election year so they probably dont want to jump the gun and then look foolish if nothing comes up in these investigations.
Here's my opinion: Cloud is absolutely right that Pence or Ryan or Hatch or anyone else would sign the tax cuts just as quickly. The GOP establishment knows that, but they're afraid of Trump's voters. They've spent four decades using hate radio and Fake News to cultivate this mob of racist, heavily-armed religious kooks who are convinced that the dam librals are out to steal their guns and Bables and make their sons gay marry Mexican immigrants then let Muslins kill them all. The human dumpster fire has them convinced that he's their Messiah; if Dump is removed from power and the GOP establishment looks culpable in any way this mob--or, as you can also call it, the Republican base--will be livid. The GOP establishment--the economic royalists who actually don't give a flying fuck about gay marriage or abortion or Jesus returning and just want their tax cuts, dammit--need those votes to keep and stay in power.
At best, the base stops voting. Worse, they vote for every bomb-throwing nut job or con-man who rails about the Establishment and invokes Dump as their martyr and primaries the more royalist candidates. In both those cases, the Democrats could benefit (See the Todd Akin/Roy Moore effect). At worst...well, we have heavily armed rioters that Fox Noise will still call "peaceful protestors" even when they start killing people.
So I wouldn't be surprised if some of them are actually hoping that the Democrats take power and get rid of Trump; their hands would be "clean", and ironically they would--without the slightest trace of shame--campaign on Dump's martyrdom in 2020 or whatever themselves.