Farming is very time efficient, if that answers your question. Like I said, you will spend 5-10 minutes to do all your farming for the day, I don't think there is anything else that you can do that fast that is that profitable.
In other words, farming is great if you consider profit/time invested... however you can't invest more than a few minutes into it because you have to wait till all the plants/livestock grow before you can "work" again.
Is it profitable to the point some people will have all their alts get islands and give co-ownership to the main?
Just for the islands I don't think so.
If you already have alts for gathering/crafting I think it's a good idea to share islands and make an extra buck from the multiple islands, but buying alts JUST for the extra farming I think is not worth it. Buying and upgrading the island and the buildings is quite expensive, doing that in multiple islands just for the farms would be a nightmare.
I hope they implement a better farming system in the future, then players can dedicate more time to that and live as farmers.