kinda the point - if you've got a bajillion users who are using the thing you're hoping to change, you have a strong limitation that you can't change anything that they're currently using. You have to maintain backward compatibility.

This is why "referer" is still mis-spelled in HTTP. To correct it without causing problems, you'd need to alter the code on every router on the planet, simultaneously. It would literally be easier to change the dictionary and every document with the word "referrer" in it.

So, yeah, there's no sunk cost here. It's more like sunk foundations - you're building on top of what has gone before, and you can only build up.