How likely is rib warp or cartilage shrinking?

I'll try to clarifiy concepts I think may be of the greatest help for you and readers:
-rib
cartilage is an excellen kind of graft, has no substitute whenever the
surgeon needs thick, strong or long grafts or when other donor sites are
for whatsoever reasons unavailable; however it is not the first choice
in structure and revision rhinoplasties in the vast majority of cases
(its morbidity involves a visible scar at the chest a a somehow painful
couple of weeks after surgery, besides the time consuming procedure to
harvest it and the technical skills required)
-in afroamerican and
asian rhinoplasties of primary type rib cartilage is required in 90% of
cases; in revision rhinoplasties (non ethnic) the percentage of rib
cartilage usage is way smaller, I'd rate it 20-30% cases
-genuine or predominantly african and asian noses the use of rib can be cosidered the rule, so in your case it was good decision
-harvesting
a rib cartilage is technically challenging, the surgeon has to be
careful not to produce a pneumothorax or a pleural tear, and its
tayloring is tricky: only the core, center or shaft of the rib cartilage
is usable, since it is the straight part which remains straight, the
cortex of the rib cartilage warps and has to be discarded as first
instance (although there are trick to use it like joining 2 warped
cortex portions facing each other like parenthesis); as a rule and
regarding the build up of a dorsum in afroamericans or asians, or saddle
nose deformities in revisions, the shaft of the cartilage is the block
we shape and taylor for the nose; failing to use ONLY the shaft leads to
warping, this is a well known fact many decades ago and still some
surgeons fail to comply with this technical standardSeek the advice and hands of a serious professional who offers to you a time-consuming and expensive procedure; stay far from minimally invassive snake-charming procedures, synthetic implants, fillers, wrapped diced cartilage, etc.
See the link below to find few cases of my own practice very similar to yours which I had the opportunity to operate successfully on, sharing a lot of common features with the technical problem you have posted.
If you wish better grounded opinion well lit, focused and standard images have to be assessed: frontal, both lateral and both oblique views, also from underneath.

Rib cartilage is the ideal source for obtaining additional cartilage to strengthen the underlying nasal structure when septum is no longer available or insufficient in quantity. Ear cartilage is also an option. Rib cartilage warps very little over time and seems to occur in about 1.5 to 2% of all rhinoplasties where this type of graft is used. Good technique is important as well as proper placement.  I personally have not had any patients warp their cartilage over time.

Cartilage grafts are the ideal grafting material. They do need to revascularize, and it is extremely rare to see shrinkage if taken from the patient's own rib.  Warping can be minimized with proper technique but hard to completely eliminate in all cases.  For a variety of reasons including warping, wrapping finely diced cartilage in a fascia wrap offers advantages in many cases.

There is always the possibility of cartilage warping, but there are several ways to control and reduce the chances of this happening.  This has to do with the way the cartilage is is cut and fashioned, and how the grafts are placed at the time of the procedure.  Choosing a plastic surgeon who has experience and expertise in revision rhinoplasty is also extremely important because he or she will understand all of the nuance when it comes to preventing cartilage warping.