is for blocks to be full so that transactions bid for space with transaction fees.
Ultimately, transaction fees alone will pay for the security. If nobody pays transaction fees (because they don't have to), then there will be no incentives to mine and and 51% attack will be possible.
Someone might actually think this through to conclusion one of these days and stop spouting the same tired old nonsense. When the time finally comes that there's no more block rewards and the network has to survive on fees alone, it will need to have far more users than we're currently capable of supporting. More users = more fees. It's not rocket science. The fees generated by the current number of users would be entirely insufficient even to be considered an incentive at all, let alone a good incentive. The userbase
must increase as the block reward diminishes.
If the intention is to have a slow network where your transaction may not get included in the next block even when paying a fee, there will be even less incentive to mine when people switch to another coin that does confirm their transactions in good time. If anything, full blocks will result in a smaller userbase and less fees to support the network. I just can't take people seriously when they say full blocks will somehow add security. Please stop.
Simply increasing the user base is not sufficient.
As the subsidy diminishes, if there is no restriction on the number of transactions in a block, then a miner will include
any transaction with a fee* because excluding a transaction paying a fee will lower their profit. That means that the fee that people pay will drop to 1 satoshi because there is little incentive to pay more. Even if every block contains a million transactions, then with a 1 satoshi fee it is only worth $2.50 (at current rates) to mine.
Furthermore, Bitcoin's "slowness" may be a problem for certain types of transactions, but it is not a problem for all types. It is not necessary for everybody to adopt bitcoin for everything. Any widespread adoption whether mainstream or niche will result in a user base that it much larger than today's. Bitcoin doesn't have to be the only currency in order for it to succeed.
Finally, full blocks increase mining revenue, which makes it more expensive to launch a 51% attack. That is why full blocks increase security.
You may disagree, but calling that "nonsense" only highlights your ignorance and your arrogance.
* There may be a lower bound to transaction fees because the marginal cost to process a transaction may be more than 1 satoshi.