I can sympathise with this point of view, but from a bluntly realistic perspective, I imagine that the majority of Construct users are infrequent, hobbyist devs who use the tool every now and then (I'd love the data on this, it's something that Scirra track).
With this in mind, they'd lose an awful lot of money if they swapped to monthly payments. It would be very easy to imagine a situation whereby the majority of users would by a handful of month subs a year, which would cost Scirra about a 3rd of their revenue from an annual sub model.
So the compromise then would be to dramatically increase the monthly sub cost; which wouldn't work for their market (entry level game dev and education).
The current model simply makes sense - it's a very low price compared to other services. There might be a middle ground for a sliding discount i.e
1 month = £18
3 month = £40
6 month = £55
1 year = £79
However I'm confident smarter minds than mine have considered and dismissed this; it's likely too complex, and as stated, the annual price is competitive enough.
I don't disagree really with anything you've said there except I note that many other services (I'm assuming game engines here?) can be had for free or free for hobbyist situations (Unity/UE4/Godot etc) and Gamemaker 1/2 are very competitively priced with a one off purchase (I did buy and try both and wouldn't have done that tbh if I could have just bought C3 instead).
However, I've tried the others and I prefer C2/C3 and appreciate the whole ethos of Scirra.
A suggestion specifically from my perspective;
There's a free option for C3 and that's really great but I find none of my basic projects fit within the model because I tend to need a minimum of 3 layers. Background, foreground and hud. The other restrictions are probably fine for me unless I find something I want to take further, in which case I'd be happy to pay for a subscription because I'd be using it more at that point.
I'm not asking for 3 layers on the basic free model either, if there was an extra option to open up the free version further (this model already exists for registered accounts) to small users for a one-off price. Let's call this another 'tier' in the free model rather than a different pricing model for the full version. "Freemium"? "Free plus"? (I realise you have to be careful how you name things these days!) "Basic"?
That's something I'd be happy to support because I do want to give Scirra some money, it's just subscriptions don't really work for me.