There are some websites where there are significant advantages to keeping a membership active (lots of benefits that have been discontinued for new memberships but are grandfathered in for existing ones...Amazon Prime, for example, still charges me the original--much lower--price for its annual membership). For these, it's pretty important to keep up one's membership.
Here, there is no financial cost to letting a membership lapse...either intentionally or due to a lack of advance warning from Shuffle. If the membership ends, then you can just renew easily the next time you try to log in and try to create a kingdom with more than Base cards. As such, I guess I fall into the camp of "Okay, lack of advance warning has no real-world consequences."
Actually, it is sort of a benefit. If you are an infrequent player, you'll actually save money with Shuffle's approach. You played 2 weeks ago, and your membership ended. 13 days later, you log in again to play. You can set up your new subscription then--saving you those 13 days of payments (for that time where you were not playing anyway).