I think paying for upgrades or updates through a subscription model is quite reasonable, especially when it comes to smart assets or software frameworks. Another approach is an ‘all-access subscription,’ similar to platforms like Netflix, where developers can download and use assets (such as 3D models, props, or scenarios) while their subscription is active. They can continue using these assets in the same projects even if the subscription is canceled.
To prevent misuse, platforms can limit the number of downloadable assets, preventing users from subscribing briefly just to download everything.
Subscriptions can also be beneficial for small indie developers, as they offer a lower entry cost for accessing quality assets, although the suitability of this model depends on the terms and specific nature of the assets.
Another option is to allow developers to purchase assets outright at a premium price, which would grant perpetual use and the flexibility to apply them across multiple projects. This approach gives developers long-term ownership, but in practice, quality work rarely involves reusing the same assets across multiple projects.