Dunno, $30k/mo is a lot of change, far more than you'll get working any job beyond perhaps the top consultants, barristers, lawyers, doctors (not in the UK, mind) & execs.
Certainly to the "average person" it's a shit ton of money.
You can do much more in affiliate marketing too if you have the drive, think big authority sites: money super market, poker listings, compare the market, etc etc.. They turn over 9 figures I believe, or at least all have at various different stages of their lives.
Affiliate marketing is no different to most any other business, you can make $30k/mo or you can make $300k/$3M/mo. You just have to scale it up. It's hard to make $3M/mo on your own for example. Once you start to get up to $20-$30k/mo you can begin employing people, scaling things up further and push the boundaries more. Of course that brings upon a big headache that many people that go into affiliate marketing are looking to avoid. Hiring people = more taxes, people wanting time for holidays, family problems, babies, fall outs amongst staff, staff deciding to have sex with each other and whatever else. So for most people they get to a level they are comfortable with themselves, and stop pushing from there onwards. They relax a bit, delegate some of the work to contractors and so forth.
Software faces those same problems though, and even better - you end up having to employ and deal with developers, who can be notoriously difficult at times (and it's very hard to find good ones). I'm doing a mixture, I have a "software" start-up, which is slowly growing locally and not making any significant money as of yet, but also aff marketing stuff. Aff marketing is faster to grow and brings good money quickly, software tends to take much longer to build up, sell, and you have to continuously innovate to keep adding value - whilst at risk at any stage of some new piece of tech making your software effectively redundant.
