That is for the Queen to decide, not her. There would have been a power vacuum for the throne left empty, since a lot of the houses were going back on their oaths as generations passed. Borros for one.
It's a chess game after all. It's the eyes of the people in the 7 kingdoms that need convincing, and if you can get majority and starve them out (the Greens in King's Landing) through attrition rather than so much bloodshed, that is better.
Until Punished Legolas really fucked up, however.
I'd agree with you if Rhaenys had just evac'd out of another door and flown around the dragonpit. But she chose the MOST DRAMATIC escape possible, and did it in FULL ARMOR, so from a Green perspective her stance seems really really clear. To then kinda step back and say "it wasn't my place to start your war" is just......I dunno, silly.
Lets debate what would have happened HAD she incinerated the dias.
The entire, or majority, or the Green Council...dead. Aegon II, Helaena, Aemond, dead. Rhaenys could then fly to Dragonstone, a quick trip mind you, and let Rhaenyra know. Rhaenyra, pregnant, might still be able to fly back with Daemon and her three older boys, dragonriders all, and at least lock down King's Landing, ESPECIALLY since Daemon has contacts there, whether or not Mysaria is burned out (or not working with him anymore). Helaenas kids would lilely be captured, but even if they escape they are but toddlers and where would they go? They'd have to be smuggled all the way across Westeros to Old Town.
So Rhaenyra now rules, can pin all the bloodshed on Rhaenys who then just f's off back to Driftmark WHERE SHE HAS BEEN ALMOST ALL THIS TIME ANYWAY, and poof, the realm is stable since there isn't really any contesting Rhaenyra as she is the only option by a country mile. Daemon is on a shorter leash these days so those fears seem mostly misplaced (his choking moment perhaps suggests otherwise), Corlys controls the Stepstones so it's back to the good old days under King J! Jace and Luc marry their step-sisters/cousins, Joffrey, Viserys, and Aegon III are free to shore up any shaky commitments (Lord Baratheon for starters) and all the dragons are back under one roof.