There is no pre-set limit to the amount of kUSD that can be mined. This amount is limited only by market demand for kUSD. As long as the circulating market cap of kUSD is growing, the newly minted coins contributing to this growth will be directed to the active miners.
We distinguish between ‘young’ and ‘mature’ phases of the growth of the kUSD ecosystem. In the young phase, market cap is growing and most mining earnings come from new kUSD minting. Over time, compute fees (called transaction fees, or sometimes gas fees in Ethereum) become more significant. Mining revenue thus shifts over time from new kUSD minting to compute fees on the network. So mUSD won’t ever become useless, but the source of mining rewards will slowly change over time.