Unless you're telling me that with the profit they've gained over the years, they should've been able to maintain the server now more than ever? Of course, how would you have privy of that knowledge?
I have no idea the exact profit that the game brought, but common sense tells you that while there may have only been 100-150 players on at a time, there were at least 1,000 active players that were paying for membership to play the game.
Let's say all of these players paid the MINIMAL rate of $5 USD (grandfathered price), that's $5,000 a month to keep a few computers running. No support, no updates, not even a simple server restart most of the time. Literally zero manpower for $5,000 a month profits.
The only thing I can personally think of is simply $5,000 wasn't much at all to the company in the big picture, so why bother with the hundreds of players that keep nagging for a little support here and there for the small profit of $5,000?
Have you considered the possibility that the Classic community population had plummeted to the point that the company saw fit to reasonably shut down the server?
It is in my personal opinion that the company should have left the game open, even one single server, so long as there was still a veteran player spending their time on it. Out of sheer respect to even one single player that helped get this company to the 75 MILLION euro yearly profit that it's earning today.
In any case, if they did have the codes, similar to Old School, don't you think it's possible to reclaim the server?
They do have the codes, they would not release them to allow another developer to take over sadly.
The servers are likely still there, collecting dust and powered off. I am quite confident that it would be a very simply process for Jagex to rummage out a server and boot it up again. Likely running in an hour's work, or less.
Hope this answered some of your questions.