if there's only 128 possibilities for the last word then what's the point of having one since it is easily guessed. easily brute forced.
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 missing.
I think the checksum idea is a badly implemented one. Sha256 is good for checksums why? I think bech32 has a more robust checksum thing going on but I found it impossible to find a good explanation of that made much sense.
Bech32 does have better error detection, but that doesn't make BIP-39's error detection bad.
Also the whole concept of a checksum embedded into your seed phrase is questionable since someone could write down a wrong seed phrase and the software could just correct it for them and they would never even know they were entering something wrong. i guess?
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.