You can earn extra Farm Cash by accepting some of those offers in the Market Place. I tried it, the first one I clicked had expired. Great... so I looked at the list of what else there was. There's NOTHING that I would click on - insurance quotes? Magazine subscriptions? Yuk! If there was something worth clicking on, I'd do it but it's like giving you a list of telemarketers and asking you which ones you want to call you. We all hate telemarketers so that's really NOT an option.
Once you reach level 80, which I have, you've gone as far as you can but your total experience keeps going up as you work on farms for others, etc. I would think the FC could be incorporated into that total rather than adding it to your total if and when they create a new set of levels. Then you wouldn't get more FC when the levels increase. Your account could jump up to the new level according to your experience but your FC would not change. You would start earning at that point.
In other words, your FC would go up as it always did but your level would not. If you spent it all and they introduced new levels the next day, you'd have to work a bit to get new things just like everyone else. This makes sense to me.