If you have sold 50% of your business to the VC, then you no longer have control of the business. (Seed-round investors may have stock; employees and/or cofounders certainly will.) You won't be able to close a deal that the investors don't like.
My understanding is that even if you take less, so that investors don't have outright controll, VCs will have incentive and substantial power to push small wins toward becoming either big wins or big failures. And they begin that push early, so that it's easy for VC-funded startups to end up in a grow-or-die situation, even if they theoretically might have taken a different path.
Note that there's some selection bias involved. If a VC thinks that the business is only going to be a $20m business, they won't invest. They also won't invest if the founders seem like the kind of people who will stop early. They're in the business of finding 10:1 odds on 100:1 money.
As an aside, it's a little dangerous to do valuation math like that. The valuations at A-round levels are highly speculative. If you take your $10m company and add $5m in cash, in theory it's a $15m company. But it's mainly fantasy. It's very different than a $15m operating business whose valuation is based on revenues and profits.