1. Everyone that wrote the code has probably left the company, or is working on more important projects.
2. The code was written in 90s and 00s.
3. I wouldn't be surprised if AOL owns the hardware AIM is running on. Deploying on smaller servers would mean a complete system replacement, which always requires large amounts of regression testing.
4. Big services usually have more dependencies than small services. Each dependency adds complexity and costs.
It probably is as hard to down scale an old service as it is to upscale a new service. It's probably less hard to downscale a new service than upscale a new service.
Also, if your users are dwindling (and not paying) it's less important to give that software attention.
