I understand this, these SegWit transactions are meant as anyonecanspend outputs after the de-activation, the same way as today. The only thing what protects SegWit transactions from anyonecanspend after activation is just another Soft fork which deactivates use of withness part to check signatures, users should be well aware of this and act accordingly.
What do you mean deactivation? There is no plan to deactivate segwit, why would there be? By that logic, we shouldn't have use p2sh or OP_CLTV because after activation another soft fork can be used to deactivate it and those features are gone. This is an unnecessary fear and is just spreading FUD to get people to not use segwit.
No, you can not spend normal transaction after any Soft fork without valid signature, but you can spend SegWit transaction without signature afer Soft fork, so "Literally anything can be changed with any kind of fork" is invalid statement.
You can create a transaction with the segwit outputs before the fork right now and be able to spend that without a signature. you cannot do that after the fork because after the fork you will need a signature to have that transaction considered valid.