Core dev work is *almost* orthogonal to making it go "mainstream".
It's like arguing TCP/IP developers aren't doing enough to make Facebook go viral.
You want Core devs to make sure the core consensus mechanism is humming along. Almost nothing else.
edit: The blockchain is to give a partial ordering off data published. Nothing else. It's not magic.
You're really not giving the core devs enough credit IMO by claiming their only significant contribution (even if filtered to "mainstream" promotion) is the Bitcoin Core client itself. Where would Bitcoin as a whole be without BIP32, BIP7x, mutlisig, P2SH, etc. despite that fact that some of those are only partially (if at all) implemented in Bitcoin Core (not to mention contributions to the bitcoin.org website as well)?