I'm not trying to tear down your frustrations, as I can see where you're coming from with your post.
However, with 250 planned nights, it wouldn't take much to re-earn your Diamond status...you'd still be Diamond for 200 of those 250 nights.
If you really want to move your business back to Hyatt, then I say you're in a good position to re-earn your Diamond status. Otherwise, if you've already got status elsewhere that you're just as happy with, then just continue to stay there.
I say this because that's the whole point of loyalty programmes. If you could comp status as many times as you wanted based on wherever you wanted to put your business (no matter how large an amount that business may be), then the loyality part of the loyalty programmes isn't working. The whole point of one-time comps is so that you can't hop around.
If your other elite status keeps you staying with them because Hyatt won't re-comp your Diamond membership, then the programmes are working as they were designed.
I'm not saying that Hyatt shouldn't examine your case again and perhaps help you out here to get some more revenue. But, if they don't, then it may be that they just get the short end of your business because of their lack of locations...not because they didn't try to keep it. The comped you once, you gave them business, then because of a lack of Hyatt locations you had to switch brands, you got status with the new brand, and they may end up with your business in the end.
IMO, that's life...and I'm sure Hyatt benefits similarly on other cases.
Regardless, good luck to you in your request should you appeal their decision. I really like the customer service I've received from Hyatt as Diamond, and I'd hate to see them loose potential revenue. If nothing else, though, as I mentioned earlier...just 50 of the 250 nights will get your Diamond status back, and in my opinion, 200 nights as Hyatt Diamond beats 250 nights as anything else.