When Bitcoin gets to a stage where the mined supply is basically 21m (that is, very little remaining btc left to be mined), will Bitcoin still remain a secure network?
Depending upon the price predictions the network will still be secured by the miners as they will earn huge transaction fees at that time also.The btc will become mainstream and there will lot of transactions in bitcoin network which will eventually results in more profits in the form of transaction fees at that time.The miners will not have to mine blocks using ASIC machines and will not earn block rewards but you can say transaction fees will be enough at that time to satisfy them and btc will be secured network.