With a Trump impeachment we will either get Pence as the front runner or some other candidate that will take the ticket. Both are going to be stronger contenders than the current version of Trump. Pence will get an incumbent boost and any new party will probably have the be pretty popular, either one could be a strong contender. The Democrats are doing a lot of infighting between the progressive wing and the establishment NeoLiberal wing. There is a high likelihood (50-60%) that the democratic party establishment will once again try to push through the weakest electableish candidate, the result of that could very well be another meh election where both candidates are equally blah and uninspiring. In a situation like that republicans have much better odds than in election where Trump is running and reasonable people will show up just to vote trump out.