From the miners' point of view, they want to profit from mining, so they won't include txs without enough fee, and the fees makes the network safer, so the reasons for transaction fee are actually coinside of each other.
Yes, this is understood. jonny1000 is intentionally breaking the roles and incentives down for analysis.
It is true that the fees make the network safer, but the big question is whether or not they provide the correct level of safety? What if the fees are low and the network is vulnerable? What if the fees are forced to be very high through some selection of magic numbers baked into Bitcoin but this generates too much safety (is wasteful of resources).
The eventual realisation of this study is that rules to do with fees and block scarcity do affect safety but not in a robust way. There is no price mechanism and it's easy to undercook or overcook the safety level. Given that market solutions exist "assurance contracts", it seems that the burden of proof rests with those that would jerry-rig the fee system to provide an appropriate level of safety.