That was a very long time ago and a very different situation.
The only way he’s successfully primaried is if he still hasn’t figured out how “presidenting” works by then (and that’s a necessary, but not sufficient condition). So, assuming he’s still as grossly incompetent in 2020 as he is today - his attempts to wreak havoc will be on the twitter feud level. Also, he won’t have anyone in congress - the D’s will still not support him, the R’s will be pulling toward their candidate who will be trying to desperately distinguish herself/himself from the current officeholder. That’s not a worry.
Trump would actually have a lot more than 3 months from a primary defeat to wreak havoc, 3 moths is just the lame duck period between elections and a new president. But people here seem to be assuming that Trump would gracefully concede if he lost the primary, which I think is unlikely if he still wants to be president. One thing that should make the GOP nervous about a primary challenge is that Trump very well might make an independent run if he doesn’t make the Republican ticket. I don’t see the Democrats managing to lose an election with a GOP contender and Trump splitting the ‘red’ vote between them, but it’s hard to overestimate the Democrat’s skill at losing elections.
Well, I mean the time between the GOP Convention and Election Day.
I’d be surprised if Kasich doesn’t try. He is term limited and leaves office in Jan 2019 anyway. It’s not like he’ll be building his resume in government by waiting for a 2024 run. He also is already 64. A 2024 run would have him at 72 on election day. We seem to not make as big of an age issue out of things anymore. He still has to acknowledge the risk of age related health problems that would be an issue cropping up.
Oh an he has already announced a trip to NH next month to promote his political book that’s coming out 25 APR. I’m sure a Saint Anselm College promotional visit is just part of a unique marketing strategy to make money on the book…really… what else could it be? 
Yes, it was a time where partisan loyalties were less fanatical. There were far more “swing” states in 1976 then in 2012 or 2016. So if anything, Trump getting “primaried” is even less likely in 2020.
And yet Trump has gone out of his way this week to piss off and alienate the Tea Party/Freedom Caucus, the very people who made “primaried” into the verb it is today. It’s like he’s trying to get impeached.
If I didn’t fear for the future of peaceful American democracy, I’d think it was delicious.