I'm positive that Daniels is not saying that a training program needs to be limited to 24 weeks. He states that he organizes it this way because his book is being aimed at an American reader who is probably looking to fit his theories into a scholastic/collegiate season.
"In figure 8.1 I show four 6-week blocks of training (24 weeks) leading up to the peak period. This is a desirable amount of time to set aside for preparation, but is not necessarily workable within the framework of a school program. School seasons often are more likely to cater to a 12-week program" (DRF 1st ed., pp 127-28).
Also note, Daniels's phase 2 - if counted as part of his "base" - does contain weekly (informal) "I" intensity work, in essential contrast to Lydiard's base.