2 things:
1) ICD-10 is much, much harder than ICD-9 was. I've coded in both and have taken classes in both. Eventually, coding in ICD-10 will be old hat, but...
2) If you ultimately play out where the industry is going in this regard, ICD-10 will eventually become ICD-11, and ICD-11 will become SNOMED CT. SNOMED CT is the most comprehensive data set that will help medicine as it transitions more into population health capitation reimbursement vs. the fee for service model that we use today. The rub is that to be able "code" in SNOMED, the computer has to do the majority of the work.
I was at a conference a couple of years ago where one of the featured speakers was discussing this topic and the fact that medical coders will ultimately be needed for several more years, but it's probably not a good gig for younger folks to get into because "coders" will ultimately really move upstream in the process and become clinical documentation specialists that work with physicians prior to discharge to ensure clean documentation for electronic processing for SNOMED CT.
I could probably geek out on this topic way beyond this, but suffice it to say, it may be a decent side gig if you get credentialed and become a per diem remote coder that gets paid a flat rate for a day's work, regardless of volume, and you stumble into a specialty that may have a low volume at that site.
I will say that the market, as of today, is in the middle of a bubble because folks ramped up heavy demand for ICD-10 augmented services, and when that levels off a bit, there's going to be a lot of coders floating around in the market place looking for contracts. You can probably start your own LLC, but there are a number of existing companies out there today that have these coders under contract, and their booking, billing, etc. operations are pretty easy to deal with, and their coders are already well versed in how to use most of the major systems (i.e. Cerner, Epic, Allscripts, AthenaHealth, Meditech, etc.). Your LLC would struggle because while you would know how to code, your productivity numbers would suffer because you probably don't have folks that will know how to navigate the existing EMR system to find all of the necessary documentation and orders needed to code.