Just making it a conversation,
It is possibly only time until the Power and Crows eventually win it,
Or is it made impossible for that to happen? If they were in the competition indefinitely, it would only be a matter of time 'til they won it. But they won't be, one way or t'other—within a couple of decades or so, either they will leave, or the competition will transmogrify into something not widely recognised as the SANFL any more.
It's far from impossible for one of the AFL clubs' reserves to win a SANFL flag. 18 months ago, one of them was only a kick away from doing it.
It's just that the odds are gamed by the fact that AFL reserves sides, playing against firsts teams of their opponents, will tend to underperform in finals compared with regular season. There are now enough AFL reserves teams that have played enough seasons in enough state leagues, that the records are there. There are other posts on the topic that I couldn't be bothered digging up now, but there are some striking examples in NSW alone both from the Swans reserves and in recent years the GWS reserves, where regularly repeated 100 point+ smashings of opponents in the H&A season segued into 'surprising' finals losses.
There are multiple reasons for this. A major one is what's technically known as 'giving a shit'. A second is that players' bodies wear down over the course of a year of AFL footy, increasing injuries and 'resting' for older AFL players at the tail end of the season, and the club is automatically taking the best and most in-form players out of the reserves to replace them. Third, very significant where it applies, is the fact that (in non-GFs) it's generally in the AFL club's interest for their state-league team to lose if their AFL side is out, so their young players commence their off-season break sooner (and so get back to pre-season training for the next year sooner, after their minimum break under the AFL players' award). Fourth are the state league finals qualification rules which have a slightly limiting effect on the reserves sides' September strength.
Adelaide reserves will no doubt hand out some more smashings in the H&A season. Unless the AFC has an exceptionally lucky run with injuries all year, I wouldn't get on them to win the flag, though.