About 6 years ago, the French expanded the area that producers could buy their grapes from and still carry the word champagne on the label. I’ve been to the champagne region many times and the soil profile is generally a couple of centimetres topsoil and then underneath solid chalk. In this expanded area it is topsoil and chunks of limestone but not solid chalk.
It is my humble opinion that many champagne houses that are using this lesser quality grape are diluting the taste that we have come to know and love as champagne. Some have lost that intense limey, chalky character and have become relatively bland and insipid, whilst others have retained their integrity. A few houses have made their styles noticeably sweeter to appease the Chinese and American markets.
There is also a considerable market in “parallel import” champagne where the product may be cheaper but it has not been transported under controlled temperature and so it may be compromised, ie not as fresh and lively as you have come to expect.