CAC payback matters more whenever cash, not theoretical lifetime value, is the binding constraint — which is most early-stage and bootstrapped companies. LTV:CAC tells you if a customer is profitable eventually; payback tells you how long you're funding that customer's acquisition cost out of your own cash before it's returned. A startup with runway measured in months should weight payback heavily, since a great LTV:CAC ratio with a two-year payback can still burn the company out before the value ever shows up.
Well-funded, growth-stage companies with longer runway can tolerate longer payback in exchange for better long-term LTV:CAC — the trade-off flips with your cash position.