I see two completely different things being discussed here:

1. Jump ship to AS, meaning stop flying AA metal in favor of flying AS metal.
2. Take same OW/AA/AS flights one would normally take but credit to AS "air mileage plan" instead of AAdvantage.

1. Makes no sense to most, unless you are based in SEA.

2. For those looking upgrades, it is best to earn miles/status on the airline program you fly the most, IMO.
If you buy paid J/F and not much interested in upgrades then I'd say fly whatever you want and don't stick to OW as J/F fares already come with most of the elite benefits and I would not worry about status.