1: It doesn't prove it. The only thing it proves is that you have the NFT. The only way to prove you have the degree is to contact the central authority, thereby invalidating the need for the blockchain entirely.
2: All your examples can be done easily without blockchain. Perks, discounts, coupons, tickets etc. We already do all of this just fine.
3: "They wouldn't". Lol. Why not? It takes them zero effort, it would be an automated system, and guess what, it totally destroys your whole assertation. Doesn't it? GameX would check with Blizzard for reason that Blizzard would also check with GameX.
4: Ownership with NFTs is entirely unenforceable. Contrary to your point, you are the one who actually doesn't understand NFTs, nor the real world it seems. You seem to want to turn everything into a bearer bond, when in fact nothing in real life will ever work that way.
ID's? Ok, you have some ID token that "proves" you are 21. Does that mean you are 21? No, the only thing it proves is that you have the token. In any consequential case, the entity wanting to verify your age would have to check back with a central authority to determine if it's valid or not, thereby invalidating the need for the blockchain.
Tickets? You somehow think any ticketing agency on earth will hard over all control to what happens to the tickets they have sold?
Coupons? You've seen all the legalese and prohibitions written on coupons I take it. There are more rules about what you can and can't do with a 10% Off Pizza Hut token than many contracts. Yet you think they'll start putting them on the blockchain?
You live in fantasy-land mate. No game company is going to devote a whole bunch of effort and programming time into allowing someone with assets from a competitor to use them in their game.