To be fair, Wikipedia says McDonald’s is now 80 (!!!) years old, I expect many foundational aspects of the business have changed since then. But it is wild to see particular aspects of the evolution. I wonder how much was anticipated/planned, and how much “just happened”
The company whose OG sign said "McDonald's Famous Hamburgers" the year they introduced "fast food" principles into their operations was actually built around shakes?
Ray Kroc (who is really responsible for the growth of the McDonald's franchise) was a milkshake machine salesman. I believe he was introduced to the McDonalds brothers when selling them a milkshake machine that could mix multiple individual milkshakes at once.
They did pretty much give up on the shakes not long after becoming a popular place. They were one of if not the first to switch from actual cow milk to chemical gelatin formulations.
So they really have always been struggling with the difficulties of quickly serving an ice cream product for their entire existence. That's a long time to have a thorn in your side and put up with it. 80 years according to the other comment.
Your local franchisee or municipality has made this decision for you: McDonalds corporate and every franchisee in my area employs straws that do not disintegrate.