parents point is that operating the machines regularly is of no interest to mcdonalds (likely bad margins) and its only interested in ensuring other niche fast food restos dont start to take up market share instead.

ie: if i want icecream i can get it at macdonalds, instead if DQ, unless the machines are broken. if DQ gains significant market share they "fix" the machines and DQ loses its market share.

not saying this is fact. just fleshing out parents

Where is the evidence of such a strategy though, besides the fact that machines are always broken? Even the franchisees would rather have a working machine.

There is none, but "broken McDonald's ice cream machine" is a whole category of meme.

So people start wondering my McDonald's is so plagued by machines that seem to only work on the third Thursday following the fifth month after the Lunar eclipse.

But it's likely a combination of disinterested staff and machines that require more involved maintenance.

Not to mention, there's several points of failure. Not only can various parts of the machine break; the drum that spins the mix, the compressor to keep the mix cold, leaks in various hoses and lines, but you can simply run out of mix.