If I'm understanding correctly, there's no option not to upgrade and continue using an old version of the product under the new model - the instant you stop paying, the product can no longer be used (after the month has finished). Personally this makes me feel like I don't 'own' the software anymore - instead, I'm just renting it.
I'm also interested to see if IntelliJ IDEA Ultimate will continue to offer the other language plugins (PHP, Python etc) that allows it to do what the other IDEs do under this pricing model.
I'd have no problem with this pricing approach if it was available alongside the existing system - so developers who use many different languages can use the subscription service if they feel it offers better value for them. Developers who prefer the previous model can opt to continue renewing annually to receive support and updates. I'm somewhat surprised and disappointed to see JetBrains forcing everyone down this path once their existing licenses have expired.