If Phil Jurkovec is truly the better quarterback right now, then he should be able to step up and take the starting job. There's absolutely no doubt he's a superior athlete, has significantly more arm strength, and can probably run faster. However, those three traits by themselves aren't the main determinant as to whether or not a quarterback is an ideal fit for Brian Kelly's pro-style / spread hybrid offense.
There are two questions, though, in terms of whether or not he's ready right now.
1) Is his accuracy good enough?
2) Can he make the quick correct reads from WR1 through WR4 and throw with good timing?
If the answer is "good enough" for both categories, then he should be able to claim that starting position. However, with those still being question marks, it only makes sense that Ian Book is the bona fide #1 QB at this time.
Book has demonstrated that his accuracy is excellent, his timing is excellent, he can make quick, correct reads for all four (or five) receivers, and that his arm strength is adequate enough to get the job done. That, plus he's mobile enough.
I would estimate that 90% of the playbook is available with Ian Book at the helm. The only question marks in his case would be the long range, downfield passes, since his long range arm strength is questionable (as of the end of 2018).
On the other hand, if you put Phil Jurkovec at the helm without knowing what you have, you're taking a huge risk. If he doesn't have the above two categories (accuracy, reads / timing), then we're back to having maybe 50-60% of the playbook available, like we did in 2017, and the beginning of 2018. If he does have the above two categories, then the whole playbook is definitely available. That's a lot of "if's" though, and it's up to Jurkovec himself, to show his stuff.