Memory has become one of the main bottlenecks in the artificial intelligence (AI) investment cycle, and market-share leader SK Hynix (NASDAQ: SKHY) is benefiting tremendously. The bull caseSK Hynix is the leader in the high-bandwidth memory (HBM) market, holding about 58% market share in the first quarter, according to Counterpoint. Management expects demand for HBM to continue to exceed supply for the next three years, which signals higher earnings over that time. That means the market is still pricing SK Hynix like a highly cyclical memory supplier -- not a sustainable growth story. The bear caseOne risk investors need to watch is capacity expansion by the top three memory suppliers -- Samsung, Micron, and SK Hynix.