Was it inevitable? Perhaps, however reactionary backlashs seems to happen from the majority in ancticipation of a marginalized group recieving political gains. It's likely that all trans people could have engaged in a masterclass in respectability politics (a very unreasonable demand), and for this backlash to have still happened.

As to your claim that gender was simple even five years ago. No not really. It's as simple as any identity is, which is to say that its controversal and up for debate. Take any identity: gamer, leftist, chrisitan, and there meaning will not be agreed upon universally.

Gender has always been complicated. It's just that most people weren't thinking about this stuff until recently because trans representation has increased in media.

The truth is that cisgender people have not had to consider how their gender, and really most identities are a social construct. What we are seeing now is culture shock, and if our society gets through this (which is by no means a foregone conclusions), our thoughts on gender will expand and evolve, as they have done throughout history.