I'm unsure why May risked the polls. She had parliamentary majority and that's all she needed legally to negotiate with Brussels and pass agreed outcomes into law.
I always saw her role as analogous to the 'night-watchman' in a cricket test. The night-watchman's job is to survive until stumps. No-one expects boundaries.
May was always going to be sacrificed after Brexit negotiations were concluded. Perhaps being a bookend PM was something she couldn't tolerate ...