I am going to throw out a complete, unsubstantiated hunch (and hope no one calls me on it)...
I am pretty convinced (although I do not possess conrete evidence) that at one point in time there existed a curriculum in aikido which lay the internal strength foundation. Probably, it was some of the older solo and breathing exercises that are not as popular today. Moreso, I think the curriculum was probably initially understood by students coming from others arts, thus requiring less emphasis in training. Eventually, this curriculum was isolated and pushed out of mainstream aikido.
I think you have to look at Ueshiba's history to find some very interesting facts.
1915 - Ueshiba spends about 30 days training with Takeda
1916 - Ueshiba spends about 40 days of "official" training with Takeda.
Between 1916 and 1922, Ueshiba must train either alone or with others.
Then in 1922, Ueshiba trains about 6 months with Takeda.
By 1925, most agree Ueshiba is a giant among martial artists.
We can almost discount 1915 because Ueshiba had only just met Takeda. In 1916, Takeda probably did show some things for IP/aiki. Ueshiba trains until 1922 when Takeda completes his training for aiki. 3 years later, he's a giant. Total years = 11 (including 1915). Total time with Takeda = maybe 1 year.
Takeda creates other aiki giants like Sagawa, Kodo, Hisa, etc, thus proving there is a very trainable method of learning aiki. Sagawa's father supports this as it was Sagawa's father that asked Takeda to teach him aiki instead of techniques (Transparent Power).
Was there ever a curriculum in aikido "which lay the internal strength foundation"? No, IMO, there never was.