Neither quote is about Character Advancement. The first is from the Character Creation section where it does call templates "crucial". The second quote is from the Supernatural Powers section and appears to be targeted at concepts more than templates. In any case, it's a positive "must justify" statement and not a proscriptive one.
If you move down two posts, you'll see a quote from YS92, which is indeed Advancement. It makes it very clear that a new power that takes a character to 0 refresh can only be chosen from the same, 'justified' list of powers (aka template).
Functionally, I'm not sure there's much difference between 'templates only being a starting point' and 'being able to freely add or switch out templates'. If not, are we just disagreeing over terminology?
To some extent. However, adding a new template requires changing your high concept to include the new template, and might also have some additional prerequisites, depending on the template. And in Fate, a simple change to high concept can have a significant impact. There's a huge amount of difference between these three (very simplified)characters:
HC: Pyromancer
-2 Channeling (Fire)
-2 Inhuman Strength
HC: Pyromancer Squire to the Winter Knight
-2 Channeling (Fire)
-2 Inhuman Strength
HC: Biomancy-Enhanced Pyromancer
-2 Channeling (Fire)
-2 Inhuman Strength
The first one (which I believe to violate the game design) gives no indication what the source of one of the character's powers is. We know he is a practicioner of mortal magic, but the strength has no justification.
The second one creates a link to a source of the inhuman strength. This character not only has mortal magic, but also owes a measure of allegiance to the Winter Court. His high concept can be compelled to hex tech items, and can also be compelled to provide service to the Winter Knight (and on the positive side can be used to boost rolls that involve either Pyromancy or his role/training as a squire). Furthermore, as Squire to the Winter Knight, he has justification (and possibly even a responsibility) to begin learning other powers that a full Winter Knight must know (ie, the Knight template). In time, him might become the Winter Knight (taking on that full template and adjusting his high concept).
The third one shows that the character has been subjected to Biomancy. But from whom, given that he has no Thaumaturgical knowledge? Wardens might be very interested to find out. And Biomancy doesn't always produce results that are fully as reliable as those produced by nature (or rather, supernature). Both of these factors might produce compels.
Three characters, all with the same powers, but who play very differently. This is (in my opinion) why the template and high concept designs shouldn't be simply brushed off and ignored.
