You strawmanned GCK vs Physical.  Yes they are worse than having the full game on the cart ... did you not read "this a step above a straight digital game"? They are objectively better than a digital copy of the game @CladInShadows gets it.  Their situation and mine are very similar with 5 Switches in the house. Admittedly I am very sure this is not the norm.

You cannot sell a digital game without also selling the account it is tied to (typically against the EULA).

I remember the days of blockbuster well, I also remember that you have a limited time with the game and most likely have to make 2 trips.  One to pick it up and if you didn't call ahead, the game you want may already be rented out, then after a week or less you have to take it back.  This is not my definition of "convenient" nor most peoples, hence one of the reasons there are hardly any rental places anymore in the days of Amazon and digital delivery.

It is also too early to tell how well GCK will hold their value in the coming decades. You PRESUME they will have no value.  You could be right on that you could very well be wrong.  If the download runs through Nintendo GCK could very well be supported just like Wii downloads are, where even though the shop is shuttered, you can still download your content from the official source.

Your "I have to resell it" is just silly.  No one is putting a gun to your head.  For you, I get your view is that it is a rental but in some cases games will be too large for a cart so it is either GKC or Digital.  So again I will say: It just seems odd that you would say something sucks that you have an option to re-sell rather than be stuck with.

Giving you an option to "rent" games via GCK is better than being stuck with a digital copy. For some like CladInShadows and I they are more convenient than Nintendo's digital sharing. 

End of the day, where possible, I would rather have the full game on cart.  If that is not a possibility at least I can sell a GCK if the game sucks and is not worth my shelf space.

Edit: Yes, you can trade in physical game key cards at GameStop. These function just like standard physical cartridges, meaning you can trade them in for cash or store credit. However, digital download codes that have already been redeemed on an account hold no trade-in value.

Yes, for me, it's about convenience and having more control over a game I own.  I'm not a "collector" so I have no emotional attachment to pieces of plastic.  I just want to be able to play my own games with the highest level of convenience.  And you're right, there probably aren't many households with a lot of switches in them.  Especially Switch 2s.  But there will be.  Especially when the inevitable Switch 2 Lite comes out. I wish more companies did the Steam thing and let you share your digital libraries across accounts, but could allow only one copy a game to be played at any given time.  But until then, Nintendo has the inadequate family plan and I will continue to get GKCs.

When it comes to digital I like better how Sony has done it.  My brother-in-law can make my PS5 his primary and share his collection with me.  The loaning games between Switches in the family account is better than nothing but it isn't without steps.  What do you find inadequate about the Family Plan?  

My main gripe, I guess, is the 14 day limit. I can't think of any situation where that makes sense in a household with multiple switches.  I originally bought my daughter Animal Crossing digitally before we had multiple switches.  Now we need to keep re-sharing the game every 2 weeks.

And the need to be in close proximity to each other.  If my kid takes their switch to a friend's place, I'd like the option, as the owner of the game, to pull the game back to my account whether they are home or not. But I guess if my kid took a GKC to their friend's house, I'd have to wait until they came home too.  So that one is very minor.

I get that they are trying to prevent people from abusing the system.  But they're making it a worse experience for those who are using it legitimately.