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Reality Revision requires a crystal worth 25,000 gp.
Wish requires a diamond worth 25,000 gp.

"Grand Jewels" in the treasure tables peak at 5,000 gp.

Perhaps there is a work around for Reality Revision if a Cognizance Crystal can be used as the material component (see linked question) as Craft Cognizance Crystal allows the use of 12,500 gp of "raw materials" to create a 25,000gp crystal over 25 days of crafting. "Raw materials" have no meaningful rarity so it's just a matter of having the funds. All good there... maybe.

So, now to Wish... a diamond is specifically cited as the expensive material component. It's not diamonds plural, it is a single diamond. The most expensive diamond you can find in treasure is worth 5,000 gp. A 25,000 gp diamond is so rare it doesn't even warrant a mention in random treasure tables.

To put the rarity in focus, even in a metropolis there is only a 75% chance of finding a magic item of up to 16,000 gp value. The maximum spell level for a metropolis is 8th, so no wish casting even in the biggest population centres listed.

How does someone procure one of these unmentionably rare 25,000 gp diamonds? And if using it for a Wish spell destroys it, how are there any left in a world with any wish casters in it?

It isn't a feasible material component when it is so specific!

The wizard has clawed his/her way up 17 levels, scrimped 25,000 gp, and then is blocked by the rarity of the material component.

Sure, make it cost a lot, but let it be practical to source the materials without becoming BFFs with Dumathoin (or whatever god of gems your setting has handy).

The rules for grand jewels don't peak their value at 5,000 gp.

Grand Jewels (5,000 gp or more): clearest bright green emerald; diamond; jacinth; ruby

Grand jewels can be worth more than 5,000gp, instead of setting a cap on them the rules do the opposite and set a minimum. The lowest value diamond you can find is 5,000 gp.

Use Fabricate to convert 5 5,000gp diamonds into a single one with a 25,000gp value?

From my reading of the Fabricate spell, the caster converts "material of one sort (which in this case would be a pile of diamonds with a total value of 25kgp) into a (single) product that is of the same material" & since the "quality of items made by this spell is commensurate with the quality of material used", the value of the individual gems should be retained allowing for a single 25kgp gem to be produced. Even the most expensive 5kgp diamonds would be small enough for 5 of them to fit in a single cubic foot of space (though if you can't find five of those, use 25 1kgp diamonds - the spell doesn't care) & the finished product isn't going to be used for anything other than as a spell component, so crafting isn't an issue.

Logically, it should work (& only needs a 5th level spell to do it too!).

how are there any left in a world with any wish casters in it?

You are aware that fantasy worlds are ... fantasies? That is, they don't actually have things like economics and, you know, people.

Take your head out of the game and look at the metagame.

They have 25,000gp diamonds in exactly the locations and quantities that the Game Master decides that they have them. Having specific and expensive material components are a way for the GM to gate access to these specific PC and NPC abilities.

Some GMs will make 25,000gp diamonds as common as glass because they want their players to have easy access to these abilities. Other GMs may make them rare and precious so that players can use the ability but will go to great lengths to find another way. Still other GMs will use these as McGuffins for adventures - "The only source of such perfect diamonds is in the Lost Mine of Eee, which is, you know, lost. Oh, and infested with demons. And a wizard that can cast Wish."

Virtually no GMs use random generation for significant treasures. Once you've given a randomly generated Staff of the Magi to a 4th level wizard you learn your lesson.