That's true to a point, but it also presupposes that the history is the only thing that really matters about Disneyland.
The other parks have history too, it is just dramatically under-publicized compared to that of Disneyland, and really, even coverage of Disneyland's history drops off dramatically after Walt's passing. I think it's fascinating to see how the parks evolved over time, and what improvements were made, as well as the things they tried that didn't quite work. It's also frustrating that because Disney isn't at all interested in the history of the other parks that their stories will never properly be told, and as a result many people assume that Disneyland is the only park that matters, because that's the only park Disney bothers talking about much of the time.
I know that interest, time, and financials are limited and finite for everyone, but I really do think that there's more value in the other parks than many people recognize, and that more people here would enjoy them than they might imagine. Disneyland has a lot of interesting history and fantastic things going for it, but it's far from the only park where that's the case.