The Milner point seems backwards: for an existing investor, cash is what they already have, and the presence of an existing investor means that their investment will be pre-diluted. If "runway" is all their startup needs, any investor would be happy to provide it.
The others seem right though, and really they all just come down to "reduced risk". The big problem for investors is that they have a hard time picking winners. YC and its community have a better track record.
Helpful to the startup, not to the investor. My reply was to patrickk above who listed Milner's investment as a draw for other investors. From the investor's perspective a hungry startup is preferable. Thus startups that aren't hungry by virtue of someone else's investments are less desirable.
The fact that a startup isn't desperate makes it more desirable -> it may get to choose investors -> it ends up with top tier investors -> it is more likely to succeed (good investors are more than a check book) -> hence it's a good investment -> better for any given investor in the round (as well as the startup - it can and should be a win-win)
The others seem right though, and really they all just come down to "reduced risk". The big problem for investors is that they have a hard time picking winners. YC and its community have a better track record.