You may find the following to be interesting reading (taken from the Royal College of Obstetricians and Gynaecologists guideline on managing placenta accreta - you can find it via google if you want to read the whole thing but it's quite heavy going and aimed at a medical audience):
The diagnosis of placenta accreta is made only if the placenta fails to separate at delivery; therefore, if it comes away it is delivered as usual. If, however, it partially separates and ?partial accreta? exists, the associated blood loss can be large.Adherent portions should be left attached as trying to separate them can cause severe bleeding. In the case review mentioned above, 25 of the 60 cases had partial placental separation; three of these women needed hysterectomy for failed conservative treatment, and 12 others had secondary procedures to evacuate the uterus, which was conserved.
So basically of 25 women with bits of their placenta accreta left in, 3 went on to need a hysterectomy later down the line, 12 had other surgery to stop bleeding but kept their womb, and in 10 this approach was successful with no further intervention needed.
It sounds like your obstetrician is following advice from their professional body and trying their best to ensure that you don't automatically end up with a hysterectomy, but it is their job to explain it all to you so that you are happy with the plan. If you are meeting them again before the op, maybe ask questions about what their planned follow-up will be and how they will monitor you for complications such as bleeding and infection.
If it a really difficult one as there is no simple and easy answer and all options are tied up with potential risks. Good luck with it all, and I hope that despite what must be a worrying and anxious time you still find joy in your imminent new arrival!