Your opinion is right on spot
I think there are other reasons also.
- Honda wants to win with a unique design, this is important to them.
-Honda is the No 1 in designing and producing 4 stroke engines and they probably used their powers to create the new 4 stroke MotoGP category in order to be able to show their suppremacy.
-Soichiro Honda always had to win with a 4 stroke, hence the NR and it's troubled history.
-Nobody has ever built a V5 bike (as far as I know), so it seemed a good option for them.
- It reminds me of the NS500 V3 that Freddie Spencer used to become 500 champion after the NR failed to produce the goods.
- The rules dictate 145 kg min weight for both 4 and 5 cylinder machines
- They chose to do it because nobody else can do something like that.
Yamaha just enlarged the R7 engine and installed it in a modified YZR frame .