Honestly, not by much. There are maybe half a dozen major CAs that make up the vast majority (95%+) of certificate issuance, and that number has been shrinking as poorly run CAs have been shut down (like GeoTrust) and other CAs have gone through cycles of acquisition (like Verisign/Symantec/DigiCert).
Besides, a lot of the market share which Let's Encrypt has acquired has been by expanding the market, rather than taking it from other players. Ten years ago, less than 25% of web traffic was encrypted; now, 80-90% of it is, and a lot of that growth has been through increased availability of free or low-cost certificates.
