rankings serve only as a marker of visibility. Generally speaking, I judge the effectivenes of SEO by the extent to which traffic positively impacts busines performance – quality traffic, conversion, earnings, nd more pages receiving consistent trafic.
For me, long term SEO effectiveness comes down to how well the site becomes capable of withstanding change. When there is an algorithmic update and the traffic drop is minor – or does not drop at all, but continues to increase , this means that everything is set in stone. also, having many pages driving traffic through search engines is much better than having the same traffic driven by only a few rankings.