The subscription plans are great, because they provide a cash flow that can be planed. You can't plan sales of the tool forever, you have to expect it to be a curve of some kind. With a long tail. Subscriptions ensure you have a slightly varying plateau.
There are many great pieces of software that run on a subscription model well.
Visual Studio (yearly)
R# (tied to updates)
Telerik Controls (yearly)
This provides the product owner a verified stream of cash, that they can hire to and plan for. It also lets them give out updates without fear of giving away something for free.
It's hard when your product is mature, and no one is buying new ones, that you put all this effort into it, and see nothing in return when an update goes out, because everyone who wanted it, already has it.
You also have to think about your customers too.
For Visual Studio, when my company gives out ultimate to everyone every year, you have to remember that is a $15,000 license yearly. For construct users, I've seen posts of people who can't even afford the 100$ version.
In the end it comes down to what your customers can afford, and if your sales model will let you survive, you have to monetize somehow eventually because the first couple years, are always great, but eventually it has to wane, just due to running out of customers. One way would be to pour some effort into the store.
Unity has an asset store, Scirra should be the non-unity store for graphics, audio etc.