by running a full node you're helping to prop your investment in the coin and you should be using it to secure your own tx's; which should be a security measure you're willing to pay for. as the user base grows, if we let it, then merchant base grows with it and they will be more than willing from a security and fiduciary standpoint to increase the availability of full nodes across the network.
^This^ is key to the preservation of Bitcoin as a growing decentralised network.
Yes, miners earn rewards directly and have a estimated ROI, but the majority of full nodes are non-mining, their owners are looking at the long-game, ecosystem growth, and the current situation of be-your-own-bank, escape from CB fiat, fast economical permissionless money transfers.
Constraining volume with a block limit which does not at least increase in line with improving technology will seriously erode the confidence of non-mining node owners, who will switch off, or switch to an alternative coin. This is the real threat to decentralisation.
If average blocks were to increase to 3,5, or 10MB and at some level cause problems for the network then full node owners would rally with Core Dev to put in place a lower temporary limit, or help fund obvious scaling improvements. Bitcoin the network constrained by technical limits will bring people to work together to maintain the $4 Billion enterprise. Bitcoin constrained by an artificial limit which is not wanted by the majority of mining power and user-base will achieve the opposite and destroy co-operation, destroy the enthusiasm to work together, and increase centralisation by reducing node counts.
Edit, of relevance here too:
Raystonn nails shut the coffin of the plan to force a "settlement layer"Much like anywhere else where liquidity moves within a
system, value will move to the network of least friction. The reality right
now is it's very easy to move value from Bitcoin to another blockchain with
less friction. Because of this, there will never be a high value settlement
network created by an artificially imposed limit on transaction rate. The
value will simply bleed out of Bitcoin to alternative blockchains offering
lower fees if this is attempted. This is basic economics.