the trust comes from the number of nodes and how decentralized they are though, running one node yourself won't make your transactions more secure I think.
You are mistaken. That is exactly Adam Back's point (well, that, and if you run a node yourself
and validate your own transactions it makes everyone more secure). Read what he wrote again.
"its not just running a full-node, you have to actually use it for transactions"
What do you think he means by that?
He has also warned against renting nodes in data centers, which is exactly what you get when you pay people to do it.
I think underlying reason some people support big block-sizes is they do not consider fullnodes only being runnable in data-center a problem
Also
https://soundcloud.com/epicenterbitcoin/eb-095 (54:16)
"It's not just running a full node. If you just run a full node in a data center and you don't do any transactions, that's actually quite useless. So it's not the number of nodes, it is the amount of economic interest that is relying on those nodes and has sort of direct trust or control of them"
Interestingly I just noticed that the quote you copied above appears to be a serious misquote since it drops the part from the middle of his statement about just running a node in a data center being useless! Not your fault, coinjournal did that.