The data centers themselves with all the supporting infrastructure (telecom/power), as well as all the chip fabs needed to build the hardware, even if the hardware itself becomes obsolete / breaks down on a 4-5 year time scale.

In the same way the rights of way obtained with all the railroads were, even if the rails / engines themselves had to be replaced every decade or so

But some hardware does last quite a long while. Fiber laid from 25 years ago is still pretty useful.