It’s a reasonable request. Unity has “Pay for Upgrades”, that is the least FAB could have done… but No, they stuck with “Pay once, own forever” 
to pamper Devs. Why? So you can all make Games for them faster. It’s how Epic earn real money. But what’s happening now is those pampered Devs can’t affort cheap Assets anymore 
and if they don’t make quality Games, Epic will see their numbers go down. Ultimately, Game Customers are not going to pay 2X more for your Games. So it’s a lesson Epic (& Devs) will have to learn the hard way 