Unlikely they'll squeeze more linodes onto a physical machine. The overhead being saved isn't that enormous. Not to mention there might be more CPU, but there isn't necessarily enough memory, disk I/O, and/or network available to make it worth it. I wouldn't worry. Plus linode has a solid track record of improving user experience, not putting it at risk. If they find a way to squeeze in a little more, you won't be losing any performance.