How do I know my Aikido won't improve unless I try cross-training? I have observed numerous people who came to Aikido from other arts and had trouble because they were trying to apply what they learned elsewhere in Aikido. Kendo practitioners have a difficult time with Aikiken, Karate practicitioners are always trying to block instead of blend, etc.
Most of Morihei Ueshiba's first generation of students cross-trained - certainly all of the well-known ones did, and it didn't seem to slow them down too much. It was more or less required in order to become an uchi-deshi. Morihei Ueshiba, of course, cross trained quite a bit, as did his primary teacher, Sokaku Takeda.
In a parallel example, children raised to be bi-lingual often start out more slowly, but they tend to develop superior linguistic skills in the end - there are a number of studies supporting this.