TL;DR is at the bottom of the post
Besides plot and Toriyama's writing, I'm bringing an in-universe explanation as to why Future Gohan could never have bested the Cyborgs in the first place.
For years in Future Trunks' timeline, after the death of his father and most of those he held close to him, it was all on Son Gohan to be the one to protect Earth and his loved ones as best as he could have. His dad died when he was just around the age of nine or ten (somewhere around there), and it's never shown that he was able to access the power of a Super Saiyan at that exact time.
From what I can recall, there was never an indication as to the timeframe where Gohan ascended and became a Super Saiyan for his first time ever. It could have been soon after those tragic events happened and all of his pent up anger got him to a point where he just transformed. He was shown to be 23 years of age by the time he passed away, so we're looking at - THE MOST - 14 years since Gohan was able to become a Super Saiyan.
Anywhere from 14 years to a just a decade or even just under a decade since he was able to accomplish a feat is in the cards, yet the issue lies in the fact that Son Gohan was never able to become strong enough to defeat #17 and #18 despite being able to transform for as long as he was.
There are a few arguments as to why such was the case.
- One of them being lack of training
This isn't to say that Future Gohan never trained in the first place. It's made apparent in the bonus manga chapter that Future Gohan would often train Trunks when he could; it's only logical that Future Gohan would train in his spare time as well, but his disadvantage comes with him not having an equal training partner to be able to challenge him and push him to his limits.
It's been mentioned in Dragon Ball how having a training partner helps with one's progress in increasing their power. And it wasn't like having multiple sessions of intense training was in the cards either. Had he made too much of a ruckus through his training, he would have brought unnecessary attention unto him and Trunks from 17 and 18. In such a world, he needed to be cautious with how he went about his everyday life.
Also as for Son Gohan, the required intensity in order to push himself to break his own limits wasn't an option given to him in the first place. Trunks as a Super Saiyan wasn't remotely capable of touching his mentor, who used his base form against him (Vol.33 chapter). Gohan was effortlessly using one arm (not that he could use both if he wanted to), smiling, and telling Trunks that he was doing a good job.
Gohan in the other timeline had more of a hard time against Super Saiyan Goten at the age of seven years old when he was training for the Tenkaichi Budokai.
On the other side of things, Future Trunks was giving a serious effort and panting, showing how much was putting into trying to get a strike on Gohan.
And it could also be argued that Gohan was putting far more focus on Trunks' training than he was on his own training, seeing that Trunks was a generation after him and had the potential to be much stronger than he ever could have been.
Future Gohan's lack of intense training, alongside the unavailability of a proper training partner was an amalgamation of a hinderance to his own progression as a warrior. The same anger that brought him to amazing powers seemed to be absolutely nonexistent, and it could have been the single driving force as to why he became a Super Saiyan in the first place and nothing more.
But I still don't believe that to be the reason why Gohan wasn't strong enough to get rid of the cyborgs. Here is why I believe he failed to beat them with his own power
- His inadequate knowledge and incapability to master the Super Saiyan form like his father
Gohan in the Future Timeline lost his father at an age where he needed him most. He was trained by Piccolo more than he had ever been trained by his father, both of whom are respected fighting geniuses, but Son Gohan didn't have enough time to grow into a more efficient fighter without the proper guidance.
For one, in the future timeline, they were never warned about there being Cyborgs, which also meant that they didn't dedicate three years of onerous training in hopes of not falling victim to #17 and #18.
Secondly, it was Vegeta who figured out before anyone else that there had to be levels beyond that of a Super Saiyan. Vegeta didn't have the time to figure this out in Future Trunks' timeline because he was killed before he had the chance to come to that conclusion. After Vegeta realized that there was something beyond that of what he initially believed, it was Son Goku who connected the dots and figured out that there was simply too much drawback that came with transforming into a Super Saiyan.
Had Future Gohan been aware of how to minimize the Super Saiyan transformations shortcomings in regards to reducing the drain of stamina (and the strain that came with) while simultaneously utilizing the Super Saiyan transformation. If Future Gohan was aware of this, he'd have the same approach as his father did and would have been able to invalidate the extensive scantiness that came with being a Super Saiyan unaware of their forms weakness.
TL;DR: Future Gohan couldn't defeat the cyborgs because he never mastered Super Saiyan. With guidance from Goku, and maybe even Vegeta, Son Gohan would have found a way, but that didn't happen to be the case.
Thoughts?