Re: Will there ever be any monetary incentives to run a full node?
A properly implemented SPV client is as reliable as a full node. I don't know why, but this myth about the unreliability of SPV clients is very common in the Bitcoin community. Even more surprisingly, this myth is spread by Bitcoin developers.
This debate is not about reliability. SPVs are simply pure consumers in a network that have no role in consolidating it while this network happens to be in the verge of an alarming centralization situation. A properly implemented SPV wallet as you've already put it, is reliable enough for the average user of bitcoin as much as bitcoin network is but the million dollar question would be how much is that?
To have a deeper understanding of the importance of full-nodes you need to take another look at User Activated Soft Forks, UASFs, and the role they play in balancing the power relations in bitcoin ecosystem. Without a strong base of full-nodes bitcoin will be left in hands of a dozen of Chinese pools and ASIC manufacturers who are established entities under the ruling of Xi and his party.
Ethereum community doesn't worry about pools they have Buterin and Ethereum Foundation and their weird understanding of governance in cryptocurrency universe, bitcoin is not Ethereum though, Core Devs have no say on the fate of the network, they are not co-ordinated/hired by a central entity, none of them is worshiped as an idol or a rock star, bitcoin is not a project and doesn't have a so-called "road map". In bitcoin, we need full nodes to keep the network safe and secure, in Ethereum all they need and all they got is Buterin and his Foundation, period.
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If, for example, you decide to compensate the owners of full nodes at the expense of miners, this directly reduces the security of Bitcoin. This raises the question of how optimal a solution would be to increase the number of full nodes by reducing security.
You've argued the same way above thread, questioning the engineering tabulations and analysis behind the concerns about the number of full nodes.To be more specific, engineers are not inventors or innovators of mathematical models, they simply use established models. Cryptocurrency is a young field of study and I'm not aware of any textbook touting a related mathematical model to this specific problem but community members have every right to be concerned about the number of full nodes in the network and discuss options to boost this number because as @ranochigo has argued correctly (and you have debunked using a straw-man fallacy) there is no penalty paid for such an increase, forget about rewarding full nodes.