They're not really getting scarcer, unfortunately; there's a lot of Earth left to go around. It's just getting harder to put them right next to the cities and subdivisions that generate all the trash. This means that your profits start to drop, especially as fuel costs rise, since you have to ship the trash somewhere else. But there's always someone who can be persuaded to take it for the right price.
The main problem with burning landfill trash is that it is hugely expensive compared to petroleum. There are also substantial environmental and operating risks, all of which add to the cost, including protection for the people who'd actually be digging this stuff up. To put this in perspective, as far as I know there are no trash-mining efforts going on, despite prices for raw materials that are so high that thieves are starting to steal beer kegs and sell them for scrap. That would make much more economic sense--a refrigerator can have $150 worth of copper and aluminum in it--and it's still not worthwhile.
