Sure it's open-source and will get noticed eventually, but there will be days if not weeks of exposure.
Same thing for all those android apps you have installed. If it doesn't add permissions, you'll never get a warning that something has really changed.
Maybe companies should be mandated to update their key stores every time they change their board members, say, so that the old keys are revoked and you must take some action to "approve" of the new people running the company.
It'd be rather impractical in today's world, although maybe machine readable policy formats would make matching easier additions/deletions of key items rather than just a text hash.
Balancing the user effort of reading and flagging each policy term (and considering all the weird implications of each change) vs the utility of being notified would be tricky though.
Maybe some sort of collaborative/curated lists of 'substantial T&C changes' similar to adblocker filterlists could be produced? (Or maybe they already are?)
Pretty easy to do code obfuscation that a human will miss and automated testing will have no idea how to handle.
Despite all the alternatives that can be installed instantly?