You're assuming the breakage will be something obvious and clearly visible. No doubt many of them will be.
However, I would foresee plenty of breakages in obscure or not immediately apparent places which may not be noticed for some time. For example they may be minor (yet still very annoying for users stumbling on them) or apparent in only certain circumstances.
Well - that's kinda what testing is for. It would remain to be seen whether or not this proves to really be the intractable problem you're imagining it to be - let's not forget that any other method will also produce bugs. The question is whether they're produced at the same rate or not. I have no evidence to really say whether or not what you're imagining would be prohibitive to using this method but my (strong) opinion is that it wouldn't be (time will tell as I put this into practice though, and I'll be following up with more posts about it).
However, I would foresee plenty of breakages in obscure or not immediately apparent places which may not be noticed for some time. For example they may be minor (yet still very annoying for users stumbling on them) or apparent in only certain circumstances.