Again, X is not a price it's a demand level (for scalper tickets). Everyone can 'bid their price' and scalpers will still buy out the bottom of the auction and take control of the pricing.
Those hypothetical scalpers will indeed have control of the pricing, but they will have no customers willing to pay what they have paid, because all customers willing to pay more will have already bought tickets.
In theory, there may have been holdouts who wanted to get a better price, as you are suggesting, who then turn to the scalpers. Those holdouts are either (a) irrational for not bidding the price they were willing to pay or (b) not actually willing to pay whatever price the scalpers want to get.
By taking this strategy of picking a demand level X and buying out all the tickets once the supply is below that level, scalpers are virtually guaranteed to lose money.
For most goods, there is no demand that is independent of price.
The dutch auction doesn't solve the race aspect. There are plenty of situations where people can't buy tickets when they want - they're at work when the price is reached, driving, the webserver is at capacity, etc. Scalpers have the advantage in all these scenarios.
Secondly, people will pay more money once scarcity and FOMO kick in. I may want a ticket for $40 but scalpers knew X would happen somewhere around $50 so they bought all the tickets then. Am I not going to see the concert for $75 buying a scalper ticket? Not a straightforward decision to make! These are the sorts of inputs scalpers will consider when deciding X.
