But no, the market won't perform horribly simply because it has performed so well for the past twenty years. The market has no memory and no idea how it has performed, nor does that matter. Stocks move with the economy and on earnings growth. If both continue to be favorable for an eternity, the market would very gladly continue to grow indefinitely, for an eternity. But there are cycles.
Economies expand for decades and then finally slow or contract for a time.
And stocks go with them. There are cycles that affect entire generations. The cycles are larger than anyone could manage or ever predict, and they usually happen for the least suspected of reasons. Just two decades ago, it took nearly ten years for stocks to reach a previous high mark. Invest and enjoy the long, slow, life-long process.