I gave you the alternative: plug-ins that could be maintained by the community. They already have the module system but all the version sensitive .net features are in the core.

For Unreal, I don't think there is a solution to closed platforms if it requires a large/sophisticated team to maintain compatibility. We can only point out that any notion of "forever" is a fiction - you have got in bed with a commercial dependency you may come to regret if they choose to change the rules.