We should keep the M9 after becoming a Junior or any rank above, just like every other gun. For the thought about keeping cadets special, every item SD has is special. Taking one away for ranking up doesn’t make much sense, even if you get the P90 anyway. For example the HK417 (even if it’s bad) is special to sentinels, but any rank above can still use it. The M9 would still be special to cadets either way because it’s their lethal weapon, alongside the baton.