So looks like the CORSAIR iCUE 5000X RGB has drifted out of stock again, is the Obsidian 500D SE a worthy replacement, how do they compare to the Cooolermaster Masterboxes, which is the nearest equivalent?
I'll be honest, I'm not 100% sure how each of those compare, although I'm sure soomeone else would be able to provide some info. I just thought I'd say that from a PCS Discord Members order, it looks like once you order you can infact change to cases that are out of stock, meaning you could order with an alternative case and once you've gone into pre-production, you could switch to the 5000X. Doing that could obviously delay your order but it's a way to get around it. Alternatively, you could order a 5000D Airflow, add some LL fans from PCS then you're able to buy the front panels sperately from Corsair now, so you could order one of them and pop it on. I don't remember the exact price but I seem to remember it works out a bit more expensive that way. All of this depends a lot on how much you're wanting a 5000X, if you're really pining after one, there are a few options on how to get one, but if you're happy with other cases, it makes the process a little more simple.
buying the 5000X elsewhere and sending it in (also allows you to get the much prettier white version)
buying the 5000D Airflow and three LL120 for PCS to fit or SP120 Elite fans to fit yourself (the 5000D Airflow front panel will give much better airflow, but if you want the aesthetic you can also buy the 5000X glass front panel separately, as @JUNI0R says)
buying an alternative case, as you say. The 500D seems to be very good, though also very expensive; the Cooler Master ones seem to be adequate at least, perhaps very good