(no worries! I have no such excuse <3 )
She wasn't sure Tony understood her… or even that her suggestion was possible. Thanos was a pathological fanatic. Then again, anyone who felt such a calling to be a hero must share some of that.
In any case. It was something to discuss with the full… team.
"I cannot beat Thanos alone," said Nebula. "I will be with all of you and play my part."
She couldn't vouch for afterwards. Because she wasn't convinced there would be an afterwards. If they could take down Thanos, she would be satisfied dying for that.