Absolutely not true. Subjective, blanket statements like that may apply to many, but they certainly do not speak for all.
Our first Orlando vacation was a 100% onsite, Disney-bubble trip. We stayed at an onsite resort (POR), took Disney transportation, and never stepped foot off Disney property. We did not have a bad experience but neither were we swept off our feet with the so-called "magic".
We have vacationed in Orlando 9 times since then, with trips #11 and 12 coming up later this year. One introduction to the MAGIC of offsite and we vowed never again to stay on Disney property. Our offsite resort offers us way more space, way more in-room amenities, way more resort recreational options, way more comfort and is WAY more affordable than the small, basic rooms offered at Disney hotels (whether value or Deluxe). The "benefits" of onsite stays (such as free transportation and the ability to book the dining plan) are not benefits for our family, so we lose nothing and gain much by staying at an offsite property.
I would suspect from your post that you have never actually visited Gaylord Palms. It, indeed, is stunning. The amenities there (including an onsite water park) blow their Disney resort competitors away. The atrium is breathtaking, particularly during the holiday season. Their character breakfast is fabulous. Their Christmas events are both unique and exceptionally well carried out: you could easily spend a day there when ICE! and the associated activities are being offered. The rooms, in most cases, are "just" rooms (two beds and a bathroom, similar to Disney) but the resort itself offers so much more.
I find most of the people who denounce the value of offsite stays have not given it a try. Disney marketing does a wonderful job of convincing many folks that an offsite stay wouldn't be as magical an experience as at one of their onsite properties. In my opinion, they have cleverly convinced the public to pay more and get less. But, value is very subjective and means something different to everyone. All I know is that if onsite at Disney was our family's only option, we probably would not vacation in Orlando as often as we do.