Downloading the pruned blockchain file directly from external source
It introduces too much trust into the download source. It's the same thing as just uploading a datadir. You aren't verifying any of the blockchain history; what you download could be a fork of the blockchain. Having such a download completely defeats the point of running a full node.

Furthermore, with downloading a pruned datadir, you will have to download the chainstate. This isn't even verifiable without downloading the blockchain itself. The uploader could have modified the chainstate to include UTXOs that don't exist, and your node wouldn't know that. You could be forked onto a separate chain or just contain invalid things in the UTXO set.

At least with a full blockchain datadir download, you could ignore the given chainstate and just verify the blocks. But that's equivalent to the old bootstrap.dat download, and isn't actually faster.