Your logic is vacuous because you can use the exact same rationale to justify the opposite conclusion.
"It's always good to buy when sentiment is good. If the bullish trend continuous, then you got in early and got a nice discount on your equities. If it is the start of a major downturn then you have taken the first step towards buying at a greater and greater discount and can continue buying all the way down"
"It's always good to buy when sentiment is good. If the bullish trend continuous, then you got in early and got a nice discount on your equities. If it is the start of a major downturn then you have taken the first step towards buying at a greater and greater discount and can continue buying all the way down"