Ain't has been around since 1749, with an't and in't preceding it. It seems to have always been associated with the common people, and familiar/colloquial talk. Dickens used it a lot for that reason.
Aren't to an't makes perfect sense among people who don't pronounce their rs like certain Brits (non-rhotic).
And isn't to in't, and haven't/hasn't to hasn't are certainly no more difficult elisions to understand than Worcestershire or Cholmondly, although those have kept their spelling because they're names.
I feel like the partial ones should be strategically placed