To be frank ... it was explained in the episode that they discovered a new form of Dilithium crystals that stay stable at much higher warp frequencies.
What SF lacked was likely a substance that would allow the engines to push through the threshold.
The Voyager crew had access to the research done on Transwarp and simply with the new substance at hand, they were able to cross the threshold.
Like I said, there are numerous ways to explain away what transpired in Threshold and it would fit within the parameters of Trek universe.
The Doctor also could have witnessed a mutation that looked like evolution, but in effect perhaps it was not.
Or there were a million other factors that could have explained Tom's mutation.
Bottom line is that the episode in question is apparently not like-able to multitude of people, and that's why it was discarded ... fine, but there were numerous other episodes of Trek from other series that were far worse in comparison.
What SF lacked was likely a substance that would allow the engines to push through the threshold.
The Voyager crew had access to the research done on Transwarp and simply with the new substance at hand, they were able to cross the threshold.
Like I said, there are numerous ways to explain away what transpired in Threshold and it would fit within the parameters of Trek universe.
The Doctor also could have witnessed a mutation that looked like evolution, but in effect perhaps it was not.
Or there were a million other factors that could have explained Tom's mutation.
Bottom line is that the episode in question is apparently not like-able to multitude of people, and that's why it was discarded ... fine, but there were numerous other episodes of Trek from other series that were far worse in comparison.