"That [rocketry pioneer] Professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react -- to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools."
That's a colorful quote from a NYT editorial in the 1920s arguing that getting rockets into space would not only be impossible, but obviously so. When Musk announced his intentions to autonomously land and rapidly reuse rockets it was mostly dismissed. Certainly companies, full of world class engineers, that had been launching rockets since before he was born probably knew a bit more about what is or not possible than some programmer with an undergrad in physics and negligible real life aerospace experience, right? Then it became, 'We've looked into it already, of course. It's perhaps theoretically possible, but it's a complete waste of money and in way economical.' Then it became, 'Shit we're a decade behind technologically!'
There are a practically infinite number of ways for why any given nontrivial thing might not be possible, and quite a bit fewer ways that it can be possible. So it's generally quite easy to formulate compelling and intelligent arguments against the viability of something (not that my little paraphrasings above were intended to be intelligent). But these arguments are not necessarily as meaningful, or productive, as they might seem. Think of how much of our technology today would have seemed impossible, or extreme long-term future tech, not that long ago -- even to those most qualified to make such judgement. This of course does not mean anything is possible, but it does mean we'd likely be a much more backwards civilization if not for the headstrong visionaries among us doing what conventional knowledge told us ought not be able to be done.