you are very wrong

While you are right that AOS in itself is not a status, but a person's stay in the US is legal while in AOS. No other status is needed. However, the reason many would want to keep a valid underlying nonimmigrant status, such as H1/H4, is to prepare for the event when their 485s are rejected, then they will have a status to fall back on. But it is not required.

For example, if you are H1 and you use 485 based EAD to egage in open market work, then you are no longer in H1 status, but you can legally work(with EAD) and stay in the US until your 485 is adjudicated.