A higher priced monthly subscription would be an interesting incentive, however would likely hurt Scirra in the long term.
Hobbyist subscription tools thrive off of annual subscriptions, for the simple fact it lets them charge a reasonable fee and locks in the user for a long period of time - let's break down some maths.
Let's say the software costs £100 for an annual subscription.
The userbase is a hobbyist one, and therefore the majority of users will subscribe, use the software for a few weeks, have a few months off, a few weeks more using the software; rinse and repeat. This sporadic use could likely be recreated in maybe 3 one month bursts throughout the year.
Now let's imagine the users ask for monthly subscriptions - how much should the developer charge? Let's call it £10/month for some easy maths; so the user gets a nice 20% discount for purchasing annually, as it would cost £120 to pay for a year in monthly instalments (a good incentive).
Now we have the average user paying the developer £30 a year rather than £100 - the users are happier but now the developer has to sell 3 times as many licences to make the same margin. Will the change in subscription model triple the size of the audience? Unlikely.
An annual subscription has further benefits from a software life-cycle standpoint - by paying for a year up front, the developers are afforded more financial freedom to add new features, which results in a much more appealing product when the time comes for the user to renew their licence. If the userbase is sporadically stringing the developer along, the lifecycle will adapt to focus on short term wins, making substantial new feature dev less likely - resulting in a lower quality end product.
Scirra a small team competing against giants who have corporate buyout funding, often dancing on the razors edge of technology; they have access to users metrics and have thought long and hard about the model that would work best for them.
I'm confident that when you consider the above, you'll agree that the very affordable subscription model in place is a well founded strategy that gives both users and Scirra the flexibility they need to make the best projects possible - whether it's awesome games or the engine that powers them.