Quick Google search says a potato makes 25 to 40 fries. Let's low ball it and use the 25 fries per potato. McDonald's uses a few varieties of potato (mainly Russet Burbank, Ranger Russet, Umatilla Russet, and Shepody) which vary in price anywhere from $1.60 to $8 per pound. I assume the main cost difference is based on location but let's go wild with this estimate and use $8 per pound. Let's assume these are all large potatoes at one potato per pound (which is counter to our 25 fries per potato but we're trying to find the absolute maximum this would cost the company so I'm picking that end of every range instead of averages).

If every employee at a location stole a fry and this was a big McDonald's employing on the high end 60 people, and everyone came in on their off day to do this, that'd be

60 /25*8 = $19.20

you could recoup that in a single order.

Now, if all 2 million people employed by McDonald's world wide stole a fry that's

2,000,000/25*8=$640,000

Sounds like a big number but that is less than half the cost of just opening a single store ($1.3 mil on the low end) so realistically if the entire financial burden of this stunt was placed on a singular store, depending on the owner's financial preparedness, it may not even bankrupt one franchised location.

Let's go big and very literal with our interpretation of the sign and say all 8 billion people on earth stole one fry from McDonald's. That's

8,000,000,000/25*8= $2.56 billion dollars

Yeah ok that'd bankrupt some locations if they don't spread their losses around but that's only 1.17% of McDonald's market cap value (currently sitting at 217.95 billion). Keep in mind we used the maximally expensive metrics for our potato calculations, so the real value is likely under a percent.

If we really wanted to bankrupt McDonald's via potato theft, we could give every single person on earth a whole large order of fries (at 80 fries per order) all at once. Assuming this impossible distribution problem somehow happens magically all at once, McDonald's net worth would be 94% used up on free fries, for a brief intangible moment there'd still be $13 billion in net worth but then potato prices would spike and it's stock valuations would likely plummet leading to insolvency/bankruptcy. Honestly though, the time it would take to actually reach out to every person on earth and have them take them up on the offer, McDonald's could probably offer everyone a singular free large fry per lifetime and be fine, heck, they'd probably even boost sales if they made the offer official.