Either the company restructures its debt now, while interest rates remain low, or it risks insolvency when borrowing costs inevitably rise.

either...or 连接两个分句，每个分句内嵌入时间状语从句——while 表示当前低利率的时机窗口，when 表示未来借贷成本上升时的风险触发点。这种多层嵌套是 C1 级关联连词的典型用法。