I don't think anyone disagrees with that - including the folks who would be in favor of trading Newk if it led to a corner OF upgrade. It isn't about preferring Bumgarner over him. If you made a Fried/Allard for Bumgarner deal, it wouldn't be for him to front the regular season rotation. I'd expect that would still set up Folty, Gausman, Newk, Teheran, Bumgarner or maybe Folty, Newk, Gausman, Bumgarner, Teheran. Using him as the #4 or #5 starter would reduce the number of starts he makes and help to keep him fresher, while also not matching him up against other teams' #1 or #2 SPs all the time.
2019 may well be Bumgarner's "last stand". The hope is that (as you mention) he's a slightly above-average SP during the regular season and that you catch lightning in a bottle with him in a couple of playoff starts. Trading Fried and Allard for him would be completely about getting something for two guys who apparently don't fit anywhere in the organization's plans while they still have value. 25-30 average to above-average starts from Bumgarner would be more than you expect to get from Fried or Allard in 2019, and the draft pick that you'd get from attaching a QO to him next winter would hopefully bring you an arm that's several years away that you might have use for at some point down the road.
It's completely about what the plan for Fried and Allard is - at this point it's almost impossible to see either of them as more than depth for the Braves. When listing the order of Pitchers you realistically expect to factor in the regular rotation in 2019 and 2020, they come in at #11 and #12 - Folty, Gausman, Newk, Julio, Touki, Soroka, Gohara, Wright, Anderson, Wilson, Fried, and Allard. As of today, there's not even room for both of them in Gwinnett's rotation unless you use a 6-man rotation there in 2019. I'm all for shifting Fried to the pen (Allard doesn't have the stuff), but if people are serious about wanting to keep Freeman and pay him an extra million dollars to be less effective, he doesn't even fit in there (#10) since he'd be behind Viz, Minter, O'Day, Venters, Winkler, Biddle, Carle, Freeman, and Sobotka - and that's if they don't add any more arms (which AA still says he'd like to do). That all assumes Weigel and others (Graham, Burrows, Dayton?) don't factor in at all.