Spoilers ahead for the second episode of Agents of S.H.I.E.L.D. Season 7, called "Know Your Onions."
The final season of Agents of S.H.I.E.L.D. means that all bets are off for what can happen and who might survive to the end of the series, and traveling through time on the Zephyr time machine presents dangers the heroes haven't faced before. In the premiere, Simmons revealed that the Zephyr can only follow the enemy Chronicoms through time, and the time windows are limited. In "Know Your Onions," S.H.I.E.L.D. proved that it wasn't messing around with its rules for time travel. The agents lost Enoch in a heartbreaking twist, but it was totally necessary.
The question of whether or not they should kill Freddie to prevent the rise of Hydra became more or less moot when the agents discovered that the time window was closing in 17 minutes, so they had very little time to wrap up the action and get back to the Zephyr before it jumped in time without them. Freddie survived, leaving the timeline intact for now, and most of the good guys made it back in time. Poor Enoch was just a little bit too slow in leaving the 1931 version of Koenig behind and returning to the ship.
Honestly, even if you don't like or care about Enoch, the defeated look on his face when he realized the Zephyr was leaving without him was pretty heartbreaking, if understated. This is somebody who chose Fitz and Simmons over all his fellow Chronicoms to help them and save Earth. Simmons noted that she wouldn't be alive if not for Enoch (which is already true a couple of times over without S.H.I.E.L.D. even revealing what happened during the FitzSimmons time jump yet), and Enoch identified himself as Fitz's best friend. He didn't deserve to be left behind!
Still, I would argue that somebody needed to be left behind, and better that it was Enoch than any of the others. S.H.I.E.L.D. really needed to raise the stakes and prove to the agents that not even Simmons can control how the Zephyr travels through time, and traveling on the "tides" of time means that they have to hustle to avoid being left behind with no hope of being returned to their own time.
The loss of Enoch should make the threat real for the agents, and probably pack an emotional punch for plenty of viewers as well. S.H.I.E.L.D. isn't messing around, and bad things can happen to the good guys. Throw in the fact that this is the final season, and all bets are indeed off. With FitzSimmons separated already, S.H.I.E.L.D. can't rip them apart again for drama, and May's life is no longer in the balance. The threat of being left behind now seems to be the biggest danger other than the Chronicoms.
That said, Enoch being the one left behind is pretty ideal. If any of the others were stuck in 1931, they might not have lived long enough to even be born, let alone make it to 2020. Previous seasons proved that Enoch can take the long way to travel through time, as long as he needs to move forward rather than backward. He kept frozen Fitz safe for decades back in early Season 5 before Fitz's death and all the paradoxes, so it's entirely possible that Enoch could pop up again when the agents arrive in a later era.
S.H.I.E.L.D. delivered a heartbreaking loss in "Know Your Onions," but it was also a loss that ended with the stranded Enoch making drinks for Koenig and promising to tell him stories about S.H.I.E.L.D. and robots. This wasn't Fitz dying or Coulson succumbing to his disease or even Bobbi and Hunter departing for the spinoff that never happened. Enoch could be back sooner rather than later. It was a necessary tragedy, but not one that will have to be permanent.