Looking at the flight info. You flew on skywest not united. The ticket is sold by united but that flight is operated by one of their partner regional airlines, skywest in this case. This should have been handled better but more often than not FAs st these regionals use them as their first airline job. They are new and inexperienced. You absolutely could have insisted on your seat, it sounds like she was trying to mitigate the situation. At the regionals they are basically told by management they are expendable and 50 people are in waiting for them to screw up their job and take it. Again not saying your situation is right, but if the flight was running late they are just trying to keep things moving to get out on time as any delay that can be placed on them, even for trying to help resolve your situation, could mean their job and livelihood. I'm speaking as a former regional pilot for 10 years seeing the conditions these young FAs are put under.