That's debatable. Some people think that the slow pace actually encourages new players to try the game, by lowering the perceived bar to entry.
It certainly gives the game a clearer identity, which can't hurt in that regard. But it's not a driving force, by itself.
And, D&D had a complicated market presence as its sales were growing by leaps and bounds: in the early 80s, the original game was still in print, there were supplements being actively produced for it, including 3pp from Judges Guild and Arduin, and there were older Basic Sets floating around, and B/X, and AD&D. So while a muddled market presence may not hurt, it clearly couldn't hold the game back either.
personally, I think "getting new players to try the game" is more of a personal networking thing--but it's certainly not an agreed-upon fact that a slow pace of release acts against garnering new players.
Agreed. By far the best way to learn a game like 5e D&D (as with the classic D&D it so carefully resembles) is to learn it from an experienced DM, preferably with some experienced players at the table. 5e has brought back many experienced players, and encourages capable DMs to run, and run the best games possible, through DM Empowerment (OK, in theory, it also encourages pathological DMs to abuse that empowerment, but on balance I think the net effect is well into the positive side).
Also agree that the slow pace of release, even if it doesn't exactly help with attracting new players, certainly doesn't hurt. It only puts off the really hard-core-system-wonks who like having a really complex, deep, system to work with -
on the player side. That's a bad thing in the sense that 5e is trying to be all-D&Ds to all-D&Ders, and abandoning such a loyal segment of the establish fanbase is arguably giving the lie to that whole inclusiveness thing, But it's not a bad thing in terms of leaving the door open for new players, and it's a good thing when it comes to creating a strong brand identity.