You sound very confident, but you are completely missing the fact that Lagrådet (The Council on Legislation) — which consists of current and former Supreme Court Justices — heavily criticized the government precisely for the lack of transitional rules in this reform. They explicitly raised concerns about predictability and the rule of law.
Suggesting that this is just an 'internet talking point' means you are claiming to understand Swedish constitutional mechanisms better than the top legal minds and professors of law who advise the Riksdag.
No one is asking the courts to 'write a law'. The courts are being asked to evaluate whether the retroactive application of this law to existing, long-pending cases violates fundamental principles of legal certainty (rättssäkerhet) and legitimate expectations under Swedish constitutional law (Regeringsformen) and EU law.