If it happened browsers started to warn their users about third party JS doing back button history stuff, I have a hunch, that many frontendies would just shrug and tell their visitors: "Oh but for our site it is OK! Just make an exception when your browser asks!" just like we get all kinds of other web BS shoved down our throats. And when the next hyped frontend framework does such some third party integration for "better history functionality" it will become common, leading to skeptics being ridiculed for not trusting sites to handle history.