And like other time-limited services outside entertainment, the duration should be made clear. I'd personally like something as clear and blunt as:

"We guarantee access for at least X months/years after paying the license.

After service is suspended we will release all information and code necessary to set up a private server or otherwise restore function."

And for the worst kind:

"We make no guarantees of access duration, and can revoke your access immediately after paying the license.

After service is suspended we will not release information or code necessary to set up a private server or otherwise restore function."

Ideally the last type dies out completely, or becomes exceedingly rare.

These always online, server-dependent, licence-limited games are very unlike what we used to deal with; Books, DVDs, CDs, and other games on disk/cartridge or with a simple download that you can keep and use for as long as you live as long as they're still stored and in readable condition.

They're very different, and should be treated like it.

There should be a very clear visual difference when looking at the box or store page of a game that is made to simply last as long as you keep the code stored, and a game that won't. A consistent warning design. Maybe two color codes.