What actually makes large scale open source development in bitcoin land difficult is the "bitcoin-core" isn't just the "kernel", it isn't even just a node, it isn't even just a node and wallet.  It is the kernel, node, wallet, and and RPC server rolled into this single tightly coupled (never a good idea in software development) unified blob of dense code.   Now I am going so say something very heretical to the followers of the Church of Latter Day Satoshis, Satoshi wasn't a very good software developer (or cryptographer).   .  He was a brilliant "idea guy".  Bitcoin as a concept was some amazing out of the box thinking and truly innovative.  The nuts and bolts of the first client however were a complete mess.   A lot of his early decisions have really hampered current day developers.  The developers can't easily make breaking changes and they need to ensure all future versions of the software are perfectly compatible with older versions of the software.