I don't think the Nazis felt as secure from internal challenge in Germany during their rule as they appear to have been in retrospect. In any case, whether it was really unnecessary or not, they seem (from the little I've read about this) to have been past masters at controlling potential domestic opposition using the rapier rather than the bludgeon, using a combination of bribery and conciliation, isolation and media control, divide-and-conquer, obfuscation and legal hoop-jumping, and judicious applications of extreme violence. Instead of flattening German "civil society" outright, they won the tug of war with it, dragging it along with them instead of having it pull them back. There was in fact open (though of course not, in the end of the day, effective) opposition to T4 from inside Germany, notably from the Catholic Church which was one institution the Nazis had to handle with some care, and Cardinal van Galen was invoking the German Penal Code against it as late as 1941. So legal changes were probably all part of the minuet.

I also don't think it's inevitable that the T4 perpetrators would have been punished if the Nazis had lost power at some point in, say, early 1941. The western world had been increasingly heading in that direction before the war, and to a large and increasing extent it's arriving back there now, as evidenced by, for example, the mysterious absence of Down's Syndrome children in many places these days. Come to think of it, many of the people implicated in T4 didn't in fact end up facing justice after the war either.