Neil makes a very good point and ISE is not an isolated product that is going on this upward trend in resource utilisation. I can't think of a single vendor product whose memory/disk footprint is reducing. Have you looked at Cisco Prime Infrastructure lately? That makes ISE look like a walk in the park.
My opinion is that, at a superficial level, a radius server can start off being quite a simple thing and if a customer's requirements are simple (e.g. 500 devices, 802.1X EAP-PEAP, and throw in some basic PAP auth, mixed with local accounts and AD integration) then one could satisfy that with a server that needs around 4GB of RAM or less. At that point you might as well build a FreeRadius deployment and you'll be just fine. Likewise, an old ACS 5.4 server sitting in the corner doing an excellent job doesn't need more than 4GB and a mediocre CPU. We still hit 100,000 authentications a day on that single box and it's a happy little camper. It gets the job done without much fuss or fanfare.
ISE comes along with the whole kitchen sink, whether you need it or not. And all the Oracle, Java, Elastic Search baggage adds up over time. There should be an ISE lite that strips all that PxGrid/BYO/Profiling/Clustering/Posturing stuff out just provides the bare bones. But this would probably require a complete product re-write because I doubt it's that modular to allow this. It's probably not viable because ISE scales really well, and it requires a lot of infrastructure to make that scaling happen. Still, it would be nice to have a lite version that isn't able to scale (for SMB market).
