PLS chart, page-21179

"In general higher highs and lows (or their opposite) seem to have some predictive merit
Until they don't"

Yes, I agree.

"SP trend also can change suddenly without any particular external event."

No, I disagree.

SP is completely different from a wingnut spinning in zero gravity. In fact, SP cannot change (move up or down) without any particular external event. It changes because there is a transaction (buying and selling) in the first place, which is an external event. When the buying is stronger, the sp moves up. When the selling is stronger, the sp moves down. And when the buying and selling is as strong or as weak, the sp moves flat.

An uptrend happens when the buying is consistently stronger than the selling for a period of time. That buying force can get exhausted gradually because the fund is diminishing, which is also an external event, or can get turned off suddenly because of a macro event such as war or natural disaster, which is also an external event.

The external events that become factors in the movement of SP are so complex that often we cannot pinpoint the particular one that triggers the change. But to say that the change happens without any trigger is false.