If Trump is out, the GOP will be so damaged they will not be able to enact anything. I do agree the longer it drags on, the closer we get to mid-terms. That said, as fast as this thing is going, I'll be surprised if Trump is still in by early 2018
I have a high degree of confidence that Trump will last through the midterms. Here's why
Forcing him our will involve drafting articles of impeachment, BUT those articles must come from the House (currently controlled by the GOP) and by precedence would be drafted by the House Oversight Committee (chaired by Gowdy - R/SC).
Nothing to date has shown that the House is willing to tolerate articles of impeachment so far - suggesting we'd need something substantially more damning to go down this route so long as the GOP holds the majority. What that could be I'd only hazard a guess.
Even if articles were drafted, it would be followed by months of high-drama testomonies and a trial, none of which the GOP wants. They'll avoid this at all costs, up and until it become apparent that NOT doing so will cost them their seats. Unfortunately, too many are in such ridiculously safe districts that there's no reason for them to set fire to their own boat while they're still in it.
Trump could quit and resign, but this seems unlike too - while he's been infurated by circumstances thus far he seems to adore the title and is convinced not only that he's done nothing wrong, but that everything would be "great" if people would just do what he says.
The wildcard is still Mueller, and DJT's team seems busy laying the groundwork to discredit him already. FBI investigations though typically take upwards of a few years, and we're on month 2. Plus, the huge scope of this investigation on so many fronts suggests it will take longer than other investigations. Even if Mueller has found criminal wrongdoing the indictments won't come out until the end when he's confident he's learned all he can from the investigative portion. I don't expect to hear anything from Mueller until 2018 at the earliest.
Interestingly, I think the most probable scenario is a strong GOP challenger to the incumbent DJT during the 2020 primary. This is something we haven't seen in over a generation - Buchanan held a lackluster challenge to H.W. in 92, but the last time a GOP seriously challenged the incumbent president was Reagan in 1980. I think this scenario will become much more likely should the GOP lose the house in 2018 and with it whatever ability they could have had to draft legislation in committee and get sent to the WH.
If this happens get ready for a stream of attack ads from within the GOP about how Trump has been a false republican, and all the counter-punching we saw during the 2016 GOP primary taken up a new level. DJT demands loyalty above all else, so any factions that challenge him will become 'the enemy' with no holds barred. Maybe the GOP will split, which I wouldn't necessarily consider a bad thing (indeed, I think it would be great if both the Dems and Reps split into 2 or more parties).