It's also more accepting for people who have various dietary restrictions and opens up an entirely new, mediocre, but fast/cheap dining experience.
One thing to note is that after some initial offerings, Burger King has yet to really keep their plant burger in the weekly coupon deluge I get. Meaning if I want like 2 entire burger meals I can get it for under $10 with the right coupon. But if I want their Impossible Whopper I still end up paying full-price for it.
So yes, if the plant burgers were cheaper than the alternative there really wouldn't be much of a reason to eat the originals any more.