Bitcoin halving and network security

I was wondering the following about the economics of Bitcoin's network security. Miners protect the network using PoW and are awarded Bitcoin in return to cover their costs. If the amount of Bitcoin received by miners decreases over time, and the price of Bitcoin stops going up at one point, won't the network become less secure as miners will have less capital to allocate to mining?

Higher transaction fees might offset the lower block rewards, but there is no way for miners to enforce consistently higher transaction fees, so how is the security of the network guaranteed into the future?