Plan A relied on one part you are forgetting. The formula. Plan A depends on the Senior Brand "figuring out" the formula for gravity, which would allow the space stations, like the NASA building, to lift off Earth, with a self sufficient population, thus saving humanity. Coop and the junior Brand's part was finding the planet to which humanity would head towards. The Plan was for everybody to follow.
So if we find a new home, what then?
That’s the long shot. There’s Plan
A and there’s Plan B. Did you
notice anything strange about the
launch chamber ...
INT. LAUNCH FACILITY - MOMENTS LATER
Cooper cocks his head, puzzling at the VAST chamber ...
there are structures built SIDEWAYS around the CURVED walls...
This whole facility ... it’s a
vehicle? A space station?
Both. We’ve been working on it, and
others like it for twenty-five
years. Plan A.
How does it get off the Earth?
Those first gravitational anomalies
changed everything - suddenly we
knew that harnessing gravity was
real. So I started working on the
theory - and we start building this
But you haven’t solved it, yet.
That’s why there’s a Plan B.
The population bomb was only Plan B. It had no use in Plan A. Professor Brand told Coop to trust him that the formula was almost complete.
The problem is gravity. How to get
a viable amount of human life off
this planet. This is one way - Plan
B. A population bomb. Almost five
thousand fertilized eggs, weighing
in at under 900 kilos.
How could you raise them?
With equipment on board we incubate
the first ten. After that, with
surrogacy, the growth becomes
exponential - within thirty years
we might have a colony of hundreds.
The real difficulty of colonization
is genetic diversity,
This takes care of that.
Cooper looks at the equipment. Unenthusiastic.
We just give up on the people here?
That’s why Plan A’s a lot more fun.
INT. PROFESSOR BRAND’S OFFICE
Professor Brand watches Cooper as he gazes over the vast
tracts of ALGEBRA covering every available surface.
Where have you got to?
Almost? You’re asking me to hang
everything on an ’almost’?
I’m asking you to trust me.
Find us a new home. When you
return, I’ll have solved the
problem of gravity. You have my
You are right, they only find out about the three planets after going though the wormhole. Kinda. They get detailed info. NASA already had a general idea before sending them to that system:
Because you don’t have resources to
visit all twelve.
No. Data transmission back through
the wormhole is rudimentary, simple
binary ’pings’ on an annual basis
to give some clue as to which
worlds have potential. One system
One? Kind of a long shot.
One system with three potential
worlds ... no long shot.
So they knew that A) multiple planets with B) a thumbs up. There was no expectation that Coop and co would be stuck in a time slippage and loose decades. This was a last ditch effort anyway.
Of course, assume that they found a planet in 10 years and came back.
This is what you want to understand, or guess at as neither the draft or final scripts tackle it. Had they found a good planet and came back without any problems, without dropping the population bomb, what then? They go back with more people and drop the bomb then. The 10+ years isn't a problem, and with a guaranteed known good planet, they have more incentive to send people and expend the effort. Brand could even keep the lie going, saying that they need to send a colonization prep team ahead to prepare for the larger colony.
Besides, fuel and supplies aside, had the plan gone perfectly, the Endurance space ship could return and go back many times. Space travel isn't a problem for space ships. It's landing and takeoffs in atmosphere that provide the greatest strain. All those miles wouldn't wear it out, it's not a car... They explicitly planned for a return. On earth they could refuel.