Why would be only 5-10, or only 500 nodes? There is no factual data to support that claim at all.
Everyone can have a p2p client that deals with 10kb/sec, has 100 mb storage requirements, needs 1 small cpu and 256mb ram. As you go up and up in hw requirements, cost goes up, "volunteers" go down - even if userbase goes up. As you hit datacenter level requirements the number of "volunteers" starts dropping significantly because costs start running in the 5 digit, then 6 digit category and eventually you'll be paying millions.
Some of us remember 8 bit CPUs running at 4Mhz with 1k of RAM and software stored on audio cassettes. Hard drives were just something you read about. The first one I actually had (nearly 10 years later) was 40MB
In the past 10 years, I have seen 10 racks of servers in a server room disappear into a handful of Virtual hosts. I'm currently running a full node on Amazon's smallest, weakest instance type. Their cost for storing the blockchain as it currently stands is on the order of $5/month.
These are issues we should not be worrying about until they are measurably becoming an issue. Like regularly full blocks already *are*