Keep in mind. Narrative is about more than the plot. Narrative includes the whole theme of the game. The game is about the Nietzschean dilemma of becoming the monster- the comments indicate that as well as the gameplay. You discover new, horrific things about the aliens as the game goes on- and are required to "test" your own soldiers- whose equipment and armor gets less human and less comprehensible as the game goes on, to the point where a soldier in Titan armor with a Plasma Rifle is barely the same person as the person with the assault rifle and body armor. The psi-soldiers are changed completely and wind up capable of doing morally reprehensible things- including violating the free will of your enemies, arguably the greatest crime against sentience possible.
You also wind up doing morally questionable things to the aliens- it's more or less stated that you torture them to death. You also wind up butchering sentient beings- either for cybernetic parts, or for the capacity to produce interceptor parts.
Multiple bases would undermine the dilemma involved- it undermines the necessity of making the choice between becoming the monster and potentially falling at its hands by reducing the world's pressure on you to find a solution and end the crisis quickly, and by reducing the pressure on you to take the low road.
It also undermines the strategic level/base-side gameplay. If you allow the players to respond to two missions, then you have to ask: Do they play both missions, one after another, or do they play one and calculate the outcome of the other?
If they play both, then that detracts from the amount of time they spend on the base screen per amount of time they spend on the tactical gameplay. This stands the risks of making the strategic gameplay feel 'tacked on' to the tactical gameplay- and they want to make them equal partners. They would then have to either lengthen the time between abductions, or take the risk.
If they calculate the outcome of the other, then that requires them to sustain essentially a full second team of uber-level people, something that's not really possible, particularly on the higher difficulties. Again, it would require a more or less complete rethink of the game. If it's difficult or impossible to win the autocalculated battles, then people will not do it, and so it would be the same as not having it; if it's easy to win the autocalculated battles, then that induces players to put the mission they want most to do on the autocalc list for higher difficulties.
With respect to the complexity of the interceptions, the interceptors have a strategic value rather than a tactical one. The interceptions are intended to be strategic decisions- do you sacrifice or remove from service an interceptor for the sake of resources? Do you sacrifice strategic resources to increase the chance of success in the interception? Giving tactical options for the interceptions removes the deliberate feeling of helplessness that they were designed to induce. You're supposed to feel powerless when intercepting a UFO except for your ability to throw strategic resources at the problem, to help emphasize how foreign, odd, and advanced the aliens are. It's supposed to be a supremely important moment when you launch an interception- a high-stakes game of choice on par with the abductions.
In short, it was not laziness that made them make these decisions- it was a calculated, informed choice with respect to the design of the game. You can certainly disagree with it, but I think it's wrong to try to say they were lazy, or ill-informed choices.