^That is all true. You're right.
Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going... And of course, to do that, you'd first have to get to your destination by some other way, and then build the laser/particle cannon there. Which kinda defeats the purpose of a sail-ship. Maybe if it proves to be more efficient it could be used as a type of a "interstellar third rail railway" that has its power source located at the stations/planets once space travel becomes somewhat common, but it's definitely not a means of making a breakthrough. It could be used for research, like a probe, but not an actual spaceship.
Unless we make cold fusion work, or some other form of highly efficient energy production and propulsion, no luck for Newtonian spaceships nor for interstellar tourists.