If the transaction costs of Bitcoin becomes too high in the future that "nornal'' users would not be able to run lightening network nodes, I don't think that will be the major concern anymore.
Such a spike in fees would affect Bitcoin all round and users would not be able to make regular, days to day transactions on the network lowering scalability. So we will be looking at a broad solution, and not one specific to running lightening network or full nodes.
Is this really a plausible concern for the next years/decades? How could that problem be mitigated?
With a steady drop in coinbase rewards over the next 2-3 decades there would likely be increased pressure on transaction fees which could lead to a feerate spike.
One of the ways to mitigate this is off chain transactions like LN, which would lessen the burden on chain and lead to a less congested mempool. There are also more BIPs which are aimed at improving scalability.