I don't think Hashem is fearful of them achieving their aim but rather is commenting of the power of unity. Being united in one goal is a very powerful thing and is a driving force in seeing success in one's endeavours.
הן עם אחד BEHOLD, THEY ARE ONE PEOPLE — They possess all the advantage of being one people and of having one language common to all of them, and this is what they begin to do!
It was this power of togetherness that Hashem realised could not be allowed to continue.
The Chizkuni writes:
ועתה לא יבצר, “and now that they all know seventy tongues, unless they will be scattered they cannot be prevented from translating their evil design into action.” [By dispersing them, they will forget the other tongues, and this will make unity harder for them to maintain. [This interpretation is first found in B’chor shor, one of the Tossaphists, Rabbi Yoseph. Ed.]
In a similar vein, the Sforno says:
הן עם אחד, the only way I can stop them from carrying out their unanimously approved plan is to sow the seeds of discord between them. The instrument of bringing this about is to confuse their language. Basically, people’s disagreements are either religious, (philosophical) in nature, or they result from misunderstanding what they mean when they speak. These people had been of one mind in matters of religion, seeing that they had all agreed with the mistaken idolatrous philosophy shared by mankind at that time. In spite of such apparently good reasons for disagreeing with one another, they did not misunderstand each other.
Thus, Hashem had nothing to fear as no mortal man can overpower G-d. Rather it was a comment to demonstrate how when a group of people come together, acting as one unit, they will see success. Obviously though, Hashem would never let this happen in this case.