For what it's worth, I developed an exchange system (essentially rule based) using the Smartform database that turned a £200 starting bank at November 2012 into a peak balance of around £3,500 by August 2013. And I'd probably withdrawn about £700 (from memory) in smallish blocks over the months leading up to that point.
Depressingly, hubris then set in. A longish bad run of results combined with some over-aggressive staking saw me down to around £1,000 by November 2013, at which point my car's clutch went (repair bill £1,500)! I took that as a trigger to pull the plug and go and do something else for a few months.
I've gone back to the drawing board, though, and will be reactivating my subscription soon with an oddsline-based system, which I've been wanting to do for a long while.
My perspective is that racing systems design is one of my hobbies. The research and programming is the "fun" part, and I've learned a lot while doing it (trying to work with the Betfair NG-API right now, for example), but my enthusiasm waxes and wanes depending on what else I've got on. And as I've got a 2-year-old son that's normally quite a lot ...
Short answer to your question is that it profitable systems are possible, and I think my experience illustrates this, but they are not easy to find and you have to explore creative, unusual angles. If it was easy, everyone would be doing it!