They needed to have a final choice because The Catalyst cannot continue the cycle for long but Shepard cannot defeat the Reapers without the Crucible.
And ME3 was not so much on building the forces to defeat the Reapers as is to how the forces to defeat the Reapers was built….through sacrifice.

Wrong. Everything lead to the choice. The entire trilogy had moments of the themes of destroy vs control.
The entire saga was about having to make tough decisions that affect the destinies of others to pursue your goals.
The ending is about what you will sacrifice.

But that's not what Bioware was going for….they were going for a victory through sacrifice theme.
The Catalyst fits this theme.
Really, ME3 wasn't ME2 2.0 and that is fine. Fans just don't get this.