It's not so much default to, but rather see more play time. Dive will still be played, it will just not be the best comp for every point.
Also, triple tank doesn't necessarily mean static, Winston Dva Hog works well too.
Also OWL's stage 2 maps will be KR, Hollywood, Lijiang, Nepal, Hanamura, Volskaya, Gibraltar and Route 66. All of which are Rein friendly if not across the map, on specific points.
Tank heavy comps have always been Ana's strong suit, and now are Moira's as well. Zen works too cause of discord, Lucio will be main support in the majority of situations because speed is too good.
As for Mercy, she's being hunted enough as is in a dive heavy meta where she usually has vertical and horizontal outs. Playing Mercy in a static comp means Mercy doesn't move around as much, which should be an easy kill for enemy flanker(s). The other supports are all LoS healers and all have either counter-flanker or escape abilities. Zen is an exception, cause with good peels, he becomes a punish hero, plus transcendence is imo the best support ult. Whereas Mercy is just a sitting duck.
All in all, the no more Mercy insta pick allows for more support and tank variation. Only thing I'm afraid of is tank heavy meta got stale very fast when it came around the first time. But we've seen some niche comps come out here and there already so I have faith.