Or you could convince some other banks to vouch for your transactions if you vouch for theirs and then get rid of the middlemen.

No, because many of the middle men are there because you don't trust yourself.

Imagine you wanted to build a Hubspot/Triggermail/Vero type system, where you send emails to users when certain events are triggered.

Now suppose you use Sendgrid or Amazon to actually transmit your mail. You are unlikely to give very many developers actual sendgrid keys or access to call sendgrid. Most developers will call an internal API to send mail. This internal API will then perform a bunch of sanity checks - duplicate detection, rate limits, dirty word filters, etc.

Can you "cut out the middle man" by letting each internal service have their own internet connected SMTP server, and stop paying sendgrid and fire whoever builds the internal API? That would be pretty silly.

Similarly, a bank can't really get rid of the risk checks, money laundering checks, data normalization checks, etc.

Isn't this similar to the cryptocurrency Stellar

You can use Stellar to build a gateway to another store of value, including bitcoin, dollar, mpesa, rai stone or, my favorite analogy, barn doors you have sitting in a shed somewhere.