Might I make another suggestion that I have already given here: https://forums.unrealengine/showthread.php?30251-Marketplace-products-license-question&p=120675&viewfull=1#post120675 but it seems more Epic staff responds to thread.
Why not offer a similar system for assets as you provide for your own engine? Users can buy assets, plugins or whatever else for life and will get the current version as well as all previous versions. They can then decide to purchase updates for new versions at a decently lowered price. The Epic marketplace already has a good alert system in place that informs you about the compatibility of an asset with the installed engine versions and could be used for third party content as well.
In my opinion that would solve several problems. Supporting and updating content or code monthly over a long period of time costs money and if cost has to be calculated into the buying price it might become rather high. It also provides an incentive for developers to not just stop updating their work when the big first release sale is over. The buyers would not loose anything, since all sold assets are shipped with source files and they could do the updating themselves anyway. However with they can also go the easy way and purchase an update. It also means that content providers can include new features or new assets in their packs in the future and still get paid.
In the end is how most software is sold, you get the software ‘as is’ when you purchase it and have to pay when you want a new version.
Regards,
FTC