This choice was because everyone knew that Doge had been raised because of Elon's influence, so it was certain that everyone would choose him. I agree with you that there is no guarantee that Elon will remain loyal to Doge. He will do anything as long as it is profitable for his company. Remember he is a business executive and what he thinks about is profit for the company, as long as it is profitable, then he can do whatever he likes.
It's natural that DOGE comes to mind first when considering the possibility of X-payment supporting crypto payments. This belief alone can fuel DOGE price increase during this bullrun, even without official confirmation from X-payment. This is understandable given that DOGE remains the trend leader among memecoins and has Elon Musk's strong support.
Currently, I'm closely following X-payment's licensing progress. If they can launch the platform soon, we'll also have clear details about X-payment's services. Hope that crypto will be among the payment methods that X-payment accepts and supports.
I'm thinking "What if Elon Musk didn't promoted DOGE a few years ago and instead, promoted a different meme coin like SHIB for example, or maybe a different coin. Will we still think the same that that coin will be the one that will be integrated to the X platform?"
Like you said though, I also believe that DOGE will be the coin that will be integrated in his platform if it comes, but I don't think that it will be DOGE alone. There will be some coins that will be included as well. Bitcoin might be one. Other coins that have a low transaction fee might be included as well. I don't know if I'm just outdated, but is Elon Musk still posting anything related to cryptocurrency on his platform until now? Because I don't hear anything about him anymore.
Anyway, I will wait for this time to come. I don't know when it will happen, or worst case is it will not happen at all.