A subscription service seems ridiculous at first glance. If done poorly it will generate bad feelings towards the company and possibly not much revenue. If done right people might actually like it. Execution and perception of value will be very important.
What sort of content updates would STERN give a pinball? We already expect bug fixes and completed code, but if I had to pay extra for that I would be pissed.
However if you got access to entirely new game modes and rule sets and new media packages, then $20 a month doesn't sound so bad. People will pay $200 for a new sculpt in a game, so why not $200 for whole new code base?
Ultimately they are going to try and get more from the whales however they can. Whether it's subscriptions, toppers, ever-climbing LE prices, it won't stop until they've taken every last $ off the table.