I've always thought 30 was a good pace for Rez. 20 is drastic, gives you like 3 or 4 Rez per push instead of 1 or 2. That's waaaaay too good, like I have to defend off a team of 10 heroes now, I could see that getting frustrating af, I think she's fine now.
20 is stupidly short. She can already extend the fights thanks to her ult that provides sustain to her team and 20 second rez will just make her even more favourable than other healers. Blizzard want lower her pick rate so that other heroes get the chance to be equally viable. Your suggestion isn't the way how balancing works.