Worth is an entirely subjective value; discussing it does little to change what is a fundamentally personal perception.
One off payments vs subscription is ultimately an ideological debate; and one that I find people shift from left to right on as they get older. Much like taxes, the older you get the more you realise they're not only necessary but vital for society.
Anyway...
One off payments effectively floor the amount of money Scirra can earn each software cycle, sure they might make more initially, but once they've made a single sale, they've removed that consumer from the market. This is completely fine if you're selling a static product, but Scirra are selling software, software that they continuously update and improve, that dramatically changes throughout it's lifespan (Compare C2 of 4 years ago to C2 today!), and services like multiplayer servers and mobile builders.
Subscription is the only logical commerce for a product like this - to say otherwise is damaging and needlessly consumerist. To sell it at a fixed one-off price and make sound business sense Scirra would have to hack the product into modules and sell it ala carte; which is terrible.