It only fails you because of lack of colonists left on the planet. You can successfully launch a starship and not fail, provided you still have enough people left to run the colony after those 200 people on the starship leave.
I believe one reason why they didn't disable the starship components, is because the game design doesn't allow the level maker to disable a specific subtree like that. It only lets them disable technology levels. The starship components are not all seperated out into distinct levels, and so they can't be disabled without disabling other techs too. There is also no real way to rewrite the tech files to do this without changing gameplay either, since some non-starship related techs are only available at higher tech levels, after some of the initial starship components can be built.
Basically, there's not enough flexibility to do this without modifications to the game engine, or supplying extra tech files with the starship components removed.
Kind of makes you wonder why they didn't just supply extra tech files. Or, since the tech levels are only really used in the campaign levels, you could have pretty much done away with keeping the tech levels in multitek.txt the same as the campaign missions. Although, that could have been a pain to rewrite.