Bam! The company just went from being a product company to a custom engineering service. You will have to drag Product Y along with your product forever as a big lump of technical debt. After a few repeat iterations of the above, you will have a franken-product with random features hanging off of all ends, confusing to general users and impossible to maintain.
If you're a Product X company and one customer wants Product Y, stand your ground and tell them to go find Product Y somewhere else. If tons of customers want Product Y, maybe pivot and become a Product Y company and ditch Product X.