First of all. It is not just a missing last word that has 128 possibilities. Every word has 128 possibilities if it is missing, assuming that no others are also wrong or missing.
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Bech32 does have better error detection, but that doesn't make BIP-39's error detection bad and SHA-256 is an appropriate choice for a checksum.
why? why is sha-256 an appropriate choice for a checksum? it was not designed for that purpose. all it has the ability to do is detect errors but not correct them right? so how is that appropriate? not being able to correct a certain minimal number of errors. it can do zero in that regard. Quote
Embedding a checksum may not be the best solution, but it is better than nothing. Also, your assumption that the software can correct a seed phrase is wrong. The software does not have enough information and would have to resort to brute force search. And that would only be practical if you know which words are wrong or missing.
which brings us back to the original question of why not just write down your seed phrase two times and ditch the checksum altogether. Because this checksum thing can't fix things if there were too many mistakes made like writing words in the wrong order and leaving a few of them out. You would be SOL. has anyone here ever been in the situation where they needed this checksum or else all their funds were lost? 
that seems like such a remote possibility as to not even be worried about happening. you could even require a valid seed phrase to be 46 words long by just duplicating the original 23 word phrase. then the last 23 words would be your checksum for the first 23 words. and they could detect AND fix errors.
