I've done more than my share of cruise research, and that is what I've found anecdotally. I know you'll find many that agree with you on there boards, but I always roll my eyes at these declarations because in my experience the huge pricing differences are often for vastly different room types/sizes. Once you get to a 240-square foot room with a split bath, a lot of the difference vanishes. Maybe not all of it, but the ''two cruises for the price of Disney argument doesn't hold up."
Using your dates, here are a couple examples:
RCCL Harmony 6/2/19, Oceanview balcony: $5,287 (nonrefundable deposit)... probably $5,700 with refundable. BUT, that's for a 182 square foot room, compared to Disney's 241. If you don't think that extra space is worth including in your analysis for a family of four, then we disagree on principle. It's not millimeters, it's almost 60 square feet. Disney's room is 33% larger.
And remember, Royal's room will not accommodate a crib if four people are staying in it. If that's the case, you have to bump up. To get a 271 square foot room on the same ship, it's $7,738 (again, nonrefundable... probably more like $8,500 with refundable). A Junior Suite costs even more ($8,927 non-ref.). NOTE: I realize 271 is greater than 241... but it's a diminishing marginal return beyond a certain point... 241 feels a whole lot larger than 182... 271 would feel larger than 241, but that extra space isn't as important at that point.
If you get Kids Sail Free, then RCCL would be materially cheaper. But that solely depends on your dates.
NCL Breakaway 6/2/19, Oceanview balcony: $5,012.32 (includes specialty dining & open bar). 207 square foot room, so closer to Disney's than Royal's. to get a 239 square-foot Mini-Suite, it's $5,272.32. I'm not sure if that's necessary with a crib; a regular balcony may be fine.
Above, Royal does not come close. If you need space for a crib, you're paying the same as Disney prices, if not more. Norwegian actually looks like a very good deal. I'd strongly consider that Breakaway cruise for $3,000 savings. However, neither Royal nor Norwegian comes with a split bath or a tub. Is that worth $3,000? Probably not to me. Bt when you factor in the rest of how DCL is basically built for small children, I might talk myself into it. More likely I'd shoot for a less busy time and not pay the summer premium.
Can you cruise for less? Of course, DCL charges a premium. But not "always", and the examples that cite enormous savings on other lines usually are doing so without comparing apples to apples... it's not always a slam dunk. It certainly is not with Royal above. Maybe it is with Carnival, but I don't think Carnival to Disney is an apples to apples comparison, either.
I was simply giving the OP a contrarian thought to the often-repeated lines about Disney's prices... when you break everything down, I think there's a good reason for a lot of the price differences. Maybe it doesn't make up for all of the price differences, but it makes up for a lot.
I'm not here to get into an argument; I'm simply giving the OP my opinion that she isn't necessarily "missing something" when pricing other cruise lines. That's what these boards are for. The vast majority of the feedback here has been very helpful and spot on... I'm just telling her that I've had the exact same experience when pricing things out. When I compare apples to apples for my family of 4, DCL is not double the price, and sometimes it's awfully close (as in the Royal example above).