It'll be hard to call it a monopoly since there are still a number of other operators out there, and these parks don't even represent the top parks in the country.
It does suck for consumers. Less competition means less big new rides. The era of a ton of new coasters opening every year around the country is probably over. There will be a couple each year, but probably not many mega coasters anymore. B&M is going to struggle unless they change their format, which it looks like they have been...
It's all about cheap and flashy. Who cares about capacity anymore, they sell more skip the line passes with low capacity rides anyway. What is new this year is forgotten about within two years anyway. Nothing is built to last or to provide an excellent overall experience, just throw something up so the park has something to promote.
SDC may benefit actually... if they continue to focus on quality attractions versus cheap and forgettable, they will have a market edge. Combined with the resort that we assume is coming, they are turning into more and more of a destination versus cheap playground like most of the SF parks.