Nay Sayers' roadblocks:
- A starship must complete its mission within a few decades: Not necessarily the case, it can take many decades and spend decades studying the target star system. Special Relativity sets a cosmic speed limit known as the speed of light.
- A starship must carry all of its reaction mass: Nonsense, make use of the interstellar hydrogen, light sails, interstellar ramjet, electrostatic drag, mag sails for deceleration, and so on.
- A starship must always accelerate at 1g: This is wasteful, as you approach the speed of light, mass increases, and takes more energy to accelerate. Just cruise when at target speed.
- An interstellar mission must have one massive starship with a payload the size of like a Nimitz-class aircraft carrier: Bollocks, it can be an interstellar probe. Even if it were a habitat ship mission, broken up into multiple smaller spacecraft, mass is your enemy and many smaller things are way better suited for colonization than just one big thing.
- The reaction must be contained in a rocket engine: Bollocks, think about my favorite propulsion concept, Nuclear Pulse Propulsion where you ride the reaction like a wave, thus saving energy and reducing stress etc.