Using Moonlight Graham as an example, here's the lowest win total the last 10 owners to appear in the WS: 69, 90, 55 (clearly had a 5-season downswing, but it was 15 seasons previous to WS), 65 (this would be your team, only season below 70 which is impressive for 26 seasons), 68, 79, 67, 71, 71, 55 (but first season, might have been a rollover takeover, and he won 82 the next year).
I don't think any of those 10 owners got to the WS through tanking. Only one of those owners had multiple 4th-place seasons in a row. It just seems to me that, in quality worlds, the owners who make WS appearances are usually consistent playoff teams, with the occasional down or rebuilding year.
Again, not saying tanking doesn't work. I'm just saying that, if you have a quality league with an involved commish and (as you mentioned) decent MWR rules, it's pretty rare for a tanker to actually execute the long-term tanking plan to a dynastic conclusion.
In good leagues, I think trade rape is a much bigger problem.