This used to be a problem years ago, and is still somewhat a problem in inner cities. Most nodes are fiber fed now, and a node is smaller than they used to be. Sometimes servicing less than 50 customers. And if you think about it, probably only 5 of those if that are actually using their bandwidth at full capacity all of the time. So there is still quite a bit of capacity in the cable.
In short, cable internet (node wise) has a lot of bandwidth that isn't easily swallowed up in most cases.
2nd thing, yes DSL is affected by distance from the CO or a remote terminal, but also affected by the wire gauge of the plant and customer premise wiring. Not all of that ancient 60+ yr old cable in your house is bad. A lot of that older cable is a much thicker gauge than what is commonly used these days, thus less signal loss per foot.
3rd thing, most DSL plans even business do not guarantee the amount of bandwidth you'll get due to the 2nd thing I discussed, and also because they'd have to offer an SLA on that, which most don't bother. They'd rather have you purchase a T1 or some other line that is fully managed and dedicated bandwidth
4th thing, the argument that sharing your bandwidth with your neighborhood thing on cable modem is kinda stupid. It is splitting hairs saying "sharing bandwidth with your neighborhood" vs. sharing the bandwidth fed to the DSLAM in the CO. It is just further down the line. If the DSLAM is full or overloaded you can run into the same issues with capacity problems just like a cable node. It's all about how the company handles the capacity issues. Do they proactively fix it, or just wait till customers complain. In the case of VZ, they usually wait until people complain. And if you are lucky they might even move your circuit to less overloaded DSLAM or card in the DSLAM. see the dslreports vz dsl forum for numerous complaints on that.
that said, I've had both DSL and Cable (currently with cable). I would have stayed with DSL but I couldn't get the speeds I needed due to the technical limitations of ADSL distances.