The problem with this as an objection is that it isn't D&D-specific. It is true for whatever game is your first. If your first game is D&D, switching to other styles is going to be difficult. If your first game is FitD, that's going to be your formative experience, and anything else with a different process or goals of play will be weird to you.
I think this is true, and it makes me feel pretty lucky.
My brother and I benefited from:
A) Being taught we could and should DM, like, as the first lesson about RPGs, by an experienced and skilled (and unusually for the '80s, female) DM.
B) Having enough pocket money to buy multiple different RPGs.
C) Being in a city where we had multiple options for buying RPGs and saw different ones.
D) Having a loyal core group of players who would dutifully try the new RPGs we suggested to them.
So instead of being sort of tied to any one system, we basically got trained from an early age to be adaptable to a variety of different systems, none of which really seems more natural/unnatural. PtbA/FitD was definitely a mental jump, but it wasn't a hard one.
It's good to realize this because I can see it's coloured my perception of how easy it is for others to learn new systems. I do think D&D has a slightly unusual deleterious effect because in practice it is one of the harder systems on the market to learn the full ins-and-outs of, and as a result people raised solely on D&D (esp. 3E and 5E) tend to overestimate how hard it is to learn other systems.
EDIT - Re: 5E I should add that part of the issue is 5E is often promoted as being the "easiest to learn" version of D&D
, and whilst that's at least arguably true of the AD&D line of descent (1E/2E/3E/4E/5E), because it's still pretty hard to learn the whole system, even thought relatively easier, people further inflate the difficulty of learning other systems.