Agreed. I for instance haven't gotten into this problem, as the only pack I updated I actually wanted to be available for free to all previous buyers of the pack.
There should be a way to include upgrades to existing products, instead of only updating them. The difference being that by adding an upgrade, previous customers of that product need to pay another price (obviously lower than the full price of the product) to get the new added files.
This is a common practice on Unity's Assetstore, there you can either update your existing products, or you can create upgrades for them which the people who have already bought the product need to pay another price (which you set, by the way) to download the new upgrade.
I don't understand why this isn't the case here on CGTrader since this is such a general and universal practice, and I'm glad that someone has opened up this topic, because I wouldn't have thought about it until I would've needed that feature.