Obviously, the Disney and Universal theme parks can sustain being open everyday, with the number of fans that they attract. But is being open year round a good thing for parks where management might be tempted to schedule that?
Think about the time needed for maintenance and construction. Or the ability to staff a park to provide excellence service. Might some parks be better off limiting their days of operation to create a better experience for all?
And then there's also the weather, and how a park's ride line-up works in varying conditions.
I'd love to hear your thoughts on the ideal operating calendar.
Closing is a good thing.
Taking away something we Love makes us want it more.
Example - Fortnite was down for a few days during a new chapter\season - Folks went Nuts and when it returned.... We celebrated..
Chick-Fil-A closed on Sundays...
Plus knowing you have a day off no matter what -is great for the Workers..
The biggest problem with being open every day is the parks lose money operating days when they're dead. We saw a short-lived trend where Six Flags execs thought they could add more value to monthly "membership" plans by keeping the parks open in December/January, but ultimately realized the amount of money they were losing by operating the park when it was freezing cold out and no one was there was so great that it wasn't worth it. Of course Holiday in the Park is a big hit at many parks like Atlanta, San Antonio, etc. But at the parks in the north the weather proved to be too much of a detriment in regards to attendance (with the one exception of SFGADV which still closes after the first week in January, but that is an exception because its the biggest market in the country, and NYC doesn't get that cold in December compared to much of the rest of the country). SFGAm (huge Chicago/Milwaukee area market) did a full on Holiday in the Park one year, then did an abridged version, then the last year they did it it was a complete a joke (like a parking lot attraction and literally only Carousel Plaza open...so the Carousel was the only ride open), then the year after that went back to closing after Fright Fest.
Now in regards to year-round parks being open every day, again it comes down to the fact that the attendance doesn't justify the operation. SFMM tried being open every day, its so-cal so its not a weather issue, but ultimately they just lost gobs of money on traditional school days when the vast majority of people are not able to go.
In regards to Disney they have such a massive business going on that its not really feasible for them to close. They have been operating Disneyland Paris every day all winter for over 30 years even though they basically lose money all winter every year.
Also to answer your original question, "is being open every day a good thing?" My answer is no for most parks. I've seen it happen many times where a park has bad attendance, and then as the year goes on, they decide to make major budget cuts to their maintenance and operations so then on days where they actually are very busy they can't run full capacity.
Reminder that Disneyland was closed every Monday and Tuesday from 1955 through 1983. Eisner changed that by opening the park 7 days a week.