The "I only play D&D" gamers are, similarly, people I have only encountered in forums, not among gamers I have met in real life in school, stores, or at conventions.
I've been gaming for a very long time, since 1977 with OD&D -- and more or less continuously since 1980. In terms of campaigns I have played (many sessions, 15+ as part of a long-term campaign) I have played in a lot over the years. There are many more one-off or 2/3 session palate cleanser games over the course of time than are listed below -- typically between AD&D, RM, D&D, or PF campaigns. Where there is an asterisk, I have played/run in multiple campaigns over the years:
AD&D *
Traveller *
Gamma World
Top Secret
Call of Cthulhu *
James Bond 007
Rolemaster *
SpaceMaster *
Stormbringer
GURPS
D&D 3.x *
LUG Star Trek
PF1 *
Star Wars: Saga Ed
Starfinder
PF2 *
5e *
Mutants & Masterminds
Runequest
Of those which received hardcore continuous play over a ~ 5 year span or more? AD&D 1st ed, Rolemaster, D&D 3.x, PF1. The ones which weave in and out most often are Call of Cthulhu and Traveller.
It's clear that FRPG is my genre of preference, but I'm not tied to the D&D brand. I stopped AD&D from 1984 through 2000 -- when we were neck deep in Rolemaster and eschewed 1st/2nd ed. Indeed, I only played 2nd ed at conventions or as a CRPG, never in a home game campaign. We sat out 2nd ed in its entirety. It was 3rd edition which brought us back into the D&D fold, as it were.
I guess the bottom line is that I don't have much of a point of reference with those who don't have similar experiences. All of the people I have met -- including everyone I game with remotely online over the past decade have similar past RPG experiences. So.... seeing people double-down on the "I only play D&D" here is more than a little odd to me.