You will be able to find work you love. If not, you will be able to easily switch projects and find something you will love doing. You will have the flexibility to work when you need money and the flexibility to work on your own projects when you have some savings in your pockets. Or better, you can do part-time projects for clients and part-time in your own projects. You can tailor your working habits.
30-40% is also probably a bit of a stretch for the recruiter's take (former recruiter here). Those numbers and higher are certainly possible, but 20% is a bit more likely. Also, if you are getting a competitive rate for your work, it really doesn't matter what the recruiter is taking, does it? If they are able to negotiate a huge finder's fee on top of your competitive rate, that's their skill being rewarded.
Hold on - I'm not talking about "finder's fee," I'm talking about agencies that seek consultants (W2 or C2C) that take a cut of 30-50% or more in perpetuity. For example, a job advertised as "$80/hr" to the consultant, is billed to the client as maybe $130/hr or more. That keeps consultant pay suppressed. The way around that is to a) know people in companies, like CTO's; or b) have a reputation that lets you not have to go through the consulting agency (or recruiter, same thing in NYC) bidding process (or at least that's what I call it).
When I was a recruiter, if I placed a candidate for a FTE role and get say 20% fee (based on starting salary), I got that same fee whether someone remains in the job for 90 days or 90 years. I don't get additional money for each year they stay. That makes sense, because I'm not really responsible at all for that hire once it's made.
For consultants, recruiters may play some role in maintaining the relationship between the hiring client and the consultant. Not all recruiters do this, but many stay in contact with the consultant (to protect their investment) and with the client (to maintain that relationship, place more consultants, and ensure the client is satisfied with the work).
I'm not trying to defend predatory practices by recruiters, but if a recruiter is able to negotiate a large mark-up for their consultant, that's a skill that they should be rewarded for financially.
To view it another way, if I get a client to pay $130/hr for an open role, and I identify a qualified consultant willing to take that job for $70/hr, that's a combination of A) my skill as a recruiter, and B) the consultant's inability to recognize their own market value or inability to negotiate a higher hourly rate.
I don't think gouging is good for the industry overall, but if the consultant is happy at $70/hr and the client is also happy at $130/hr, what's "wrong" with the recruiter taking that $60/hr?
Everybody is happy, no? At least everybody is happy when there is no transparency as to where that $130 is going.
FYI, I'm playing devil's advocate here. During my career I did very little consulting work and my margins were usually closer to 15-20% of hourly rate, though I once worked for someone who had a 400% mark-up on a consultant for almost 3 years (consultant was paid ~$50/hr and bill rate was over $200).
I'll actually pay a finder's fee to those who find me consulting gigs (ones that pay senior rates). LinkedIn isn't working for me, here.
edit: source: I was a hiring manager in a past role and I learned the real take-home pay of the consulting agencies when they farm out consultants.