There really isn't anyone left to fight, as far as Tywin knows. Stannis is broken, nobody except Varys really knows about Dany, the Wildlings and the White Walkers are not taken seriously. Dorne is a small country without forward military power, you don't wanna invade it (its like Vietnam) but you aren't worried about them coming out and fighting everyone. The Iron Born are a problem but its a small problem that mostly falls into Roose's hands now. Who else is left?
if Marge got pregnant from Joffrey then it's a great time to kill him. Marge would be Queen Regent, her baby will be the future king, and the Tyrells can gather around and make sure nothing happens to the baby, while slowly dismantling the Lannisters from Kings Landing. With Joffrey gone and Marge not pregnant, the future is very much up in the air. Tommen won't get Marge pregnant for quite a while, so the succession is uncertain, plus leaving Tommen as king means Tywin and Cersei and Jaime all are heavily involved. Much better for Olenna to just have her daughter and (non existant) grandson be the only ones with the right to rule.
Thats my point, yeah it probably would suck for Marge for a few days to suck Joff's dick and get pregnant, but that is a much more stable situation for her family than killing off Joffrey and hoping shit doesn't go sideways over the next 5 years, waiting for Marge to get pregnant. That is totally a Littlefinger plan, and it doesn't make much sense to me that Olenna would go along with it. Or that Twin would go along with marrying Marge to Tommen. His only two real problems now are getting his idiot heir Jaime to drop the Kingsguard and take over for him, and making enough payments to the Iron Bank so they don't go help Stannis.