somewhat unnecessary if we end up lifting the message size limit.

I messages have a limit for a reason: so you can know bounds on how much you are committing to when you replicate a feed.

Blobs are a release valve to that - you can make something much bigger - but also, your followers aren't committed to replicating your blobs. Depends on the application. There are already several applications that have some sort of structured data in blobs. for example, ssb-git uses has packfiles, and ticktack has blogs in markdown format. If a blob is plaintext, I think it definitely makes sense to be able to search inside of it.

Also, searchable blobs is a pure feature - it's not removing anything, so we don't have to change how something works to add that.