To be fair, how could they have known? They worked with the data they had at the time, and hindsight is 20/20. Hard things always seem impossible until someone does it, and then everyone else talk about how it was "inevitable".
there is a difference between "impossible" and "hard". Someone comes in and pitches a 60% efficiency solar cell. I know that is an extremely hard problem, but not impossible. I'm going to grill the shit out of them over it.
Someone comes in and pitches a 110% efficiency solar cell; they aren't getting a dime of my money. Normally, they aren't coming pitching a 110% efficiency solar cell, they just haven't done the math to realize that is what they are pitching.
The assumption that needs to be challenged is that you need faster than light latencies to accomplish your objectives, not the notion that faster than light is impossible. Once physical limitations become apparent, you need to focus on the aspects of your design that are not limited by physics.
Hindsight? Grandparent told the VC that it was impossible before they decided to invest tons of money in it.
How could they have known? Acknowledging the special theory of relativity would have been a good start. It's not like they were a bunch of alchemists in the middle ages only having various mystical interpretations of the world to work from.
/s