What, exactly, do we hope to achieve from having end-to-end encryption?

Even if it worked perfectly, it wouldn't be very useful.

But it won't work perfectly, because we don't have any method of
authentication. The bitcoin network is trivially MITMable. It's designed to
work even in the face of that, but any encryption we implement will just
get blown away by anyone who cares enough to stand in the middle of two
nodes.

As far as I can see, we get a microscopic obfuscatory advantage over a very
weak passive attacker, at the cost of hugely increased software complexity
(and possibly increased CPU time).

So again; what do we hope to achieve? Why bother? Not a lot of sensitive
information is transmitted in the clear. The little information that might
be considered sensitive is better protected by anonymization (a la Tor),
not encryption.