Difficulty adjustment is a Hard Fork.
Difficulty Adjustment and Emergency Difficulty Adjustment (EDA) are 2 different thing. Only EDA which require hard-fork.
* If Attackers are earning money by doublespending, then they can continue indefinitely by using their ill-gotten gains to fund future attacks.
Double-spend isn't something you could do continuously, the receiver eventually will reject all payment/deposit and waiting the attack stopped.
Additionally, Bitcoin price would crash which makes attacker's "earned" Bitcoin have less worth.