This. My kicks (and punches) got a lot more powerful when I added heavy bag practice, which was a rare part of my early training. Makiwara, sparring, and practicing with the air - even striking into striking pads held by partners - didn't do nearly as much for power development. I'll also add that flexibility matters. I'm not highly flexible at the hips, so high kicks lose power. I can pummel a ribcage, but kicks to the head are much softer.