I don't know if this has been addressed before in this thread
I don't think adjusting the block size up or down or keeping it the same will have any effect on whether or not transaction fees will be enough to secure the network as the block subsidy goes to zero (and, as I said, I'll ask professional economists what they think).
But it's pretty simple: miners won't mine until they have enough transaction fees in a block to be able to pay what it costs them to mine. You'll have to increase the fees enough or increase adoption enough (more txs) to have a decent enough hashrate for the network to be reasonably secure.
Another side effect is that miners will have more power than they have now, they'll be able to establish cartels and request premiums for fast transactions. Right now we're used to transactions being treated more or less equally and fees being essentially zero, that equal treatment will come to an end. There is no question the block size will need to have been increased by then in order to fit a larger amount of txs; without many more txs than we have now, fees would be pretty high.
Ideally, mass adoption would be enough to keep txs cheap and net hashrate high enough. But I don't think we can expect that to happen while other issues remain unresolved. Bitcoin is not a very good payment system and the average person has no reason to adopt it yet. Some libertarians call for ignoring that and recognising Bitcoin for what it is now and its only strength: circumventing state and bank control over our money. But that is short-sighted too because Bitcoin is not designed to survive that way: without mass adoption it is just a very expensive and clunky trustless way of irreversibly sending money.