I hope you understand that "Byzantine agreement" is all there is to bitcoin, deep down.  Bitcoin is nothing else but the fact that there is an established, unique, unfalsifiable Byzantine agreement over what coins were rightfully created, and how they were transacted.  There's nothing else to it.

You are perfectly right that "all else" (that is to say, nothing of importance) is done by nodes.

Those deciding on that Byzantine agreement are hence bitcoin.

That said, there IS indeed an important point that will give true power to full mining nodes, and I neglected it to some extend: the need to keep up the belief in decentralization even when in reality it is gone.    The belief in decentralization (whether the system actually IS decentralized or not doesn't matter) is the driving force in people pumping money in this thing.  If you somehow could too obviously see that that belief is bogus (while it is, but as long as it makes for a story for the gullible, one can convince), you may kill the very religion that makes the stake holders rich.   So in a way, stake holders like miners need to PRETEND to be kept in check by nodes, so that the node owners continue to BELIEVE they have power and the belief in decentralization can be propagated.

This is a bit akin to wanting your kid to keep believing in Santa Claus because that's how you can make him behave the rest of the year.  You're obliged to play Santa Claus in order to maintain his belief, and you have to give him the presents he asked Santa, even though of course it seemed that you had the full power not to: if the belief in Santa by your kid is necessary, you have to act as if Santa were real.

In a way, then, it is true that nodes do keep miners in check, because miners have to act like they do so to keep the belief up, like you have to act like Santa exists to keep your kid believe in it.   This is a game-theoretical aspect I didn't realize.  

In as much as it is true, bitcoin is now entirely open to a Sybil attack by nodes.  Because miners have to pretend being kept in check by nodes, and nodes can easily be sybilled.  In as much as miners have to keep up the appearances, they will have to do what the sybil nodes tell them to do.  Like Dad has to do what Santa tells him - or blow the illusion apart and tell the kid that Santa doesn't exist.

(that's what I called earlier: social systems based upon lies and deceit - all social systems - eventually crumble when they hit the wall of inconsistency).