Really? Gygax outlines the importance of balance numerous times in the AD&D DMG. Most of the changes from AD&D to 3e D&D were made because of balance issues. Why do you think we got concrete rules for just about every typical action in 3e? Because leaving it up to ad hoc DM fiat resulted in all sorts of balance issues is the primary reason that was given.
Everything in D&D has always been balance and gameplay first, with realism and whatnot a secondary consideration. The only difference is that as time has gone on, we've become much better at crafting rules that come out balanced in the end.
Emphasis added. Where balance has been compromised, this has largely been to facilitate gameplay. I would not say balance has ben the only consideration, or even that it has always been the most important consideration. I would say it has always been a priority. Now, 1e relied a lot on "balance over entire campaign", which meant wizards started off very weak and grew very powerful, not really balance at most points in time with the fighter, but viewed as balanced overall. Similarly, nonhumans got a bunch of advantages offset by a fixed cap on how far they could progress. Some classes were "balance" by the difficulty in achieving the required attributes to play them.
3e changed the playing field, to me, in two ways. First, it refined balance to "balance across all levels", so balance was no longer achieve by starting weak and finishing strong (wizards) or vice versa (nonhumans), or by minimum attributes (which were eliminated), point buy introduced and variable xp per level removed. Second, it unified the mechanics and worked to reduce GM judgement calls by expanding what was covered by specific rules. But both still aimed at gameplay and balance (and, actually, heavily unbalanced games have been a turnoff for many gamers and potential gamers over the years, so balance does tend to make for better gameplay, all other things being equal).
No, I am not completely mistaken on this. Balance and game play has always been the primary motivator behind rules. This is why we don't get critical attack and fumble rules. This is why we use hit points and not a more complicated system. This is why we use an initiative system that takes only Dex into consideration when determining who goes first. This is why fighters start with more gold than clerics. On and on and on.
Agreed - I also agree balance is not the sole, or even principal, motivator - game play has always been equal or greater in importance. However, I'm hard pressed to think of anything other than playability which has commonly overridden balance in the design and redesigns.