There's a lot going on here. Partly, I think we're conflating having the best possible defense with having a fair trial. The latter probably does not require the former and, in any case, it is probably not possible under any reasonable system to provide the former for everyone. I think many defendants
do receive fair trials, whether they can afford representation or not--though this does not mean their representation could not have been better.
In fact, today, for poor defendants, the lawyers on both side are already salaried state employees. Moreover, federal defenders and some state public defenders (particularly in big cities) are actually very good. Believe it or not, these are highly coveted jobs, despite the dismal pay and stressful work. But that doesn't mean it isn't still possible to go hire an even better lawyer. And, when your life is on the line, this is just what people will do if they can possibly afford it.
This will result in disparities, but it's unclear how we could prevent this short of preventing people from hiring private attorneys. This seems like a non-starter, so it's probably better to focus our resources on making sure everyone's representation is adequate, than on making sure everyone's representation is the same.
But the other problem, of course, is that, although many public defenders are actually extremely gifted, their offices don't receive nearly enough funding. This means that, although it is possible for a given defendant to receive good representation (and some do), in the aggregate public defenders offices simply cannot properly do all the work that needs to be done. This is, in theory, an easy problem to solve, but unfortunately, there is little public interest in solving it. Prosecuting criminals is popular. Defending them is not. (Of course, the whole point is that not all defendants are criminals, and we need good lawyers to help the system distinguish between the two. But that's not the message that sticks with people, for some reason.)