If he has knowledge of the game and parents come in asking that question, 9 times out of 10 the salesman is going to point to the core 3 and leave it at that.
Maybe, maybe not. I've seen a lot of game store employees who wouldn't have the foresight to do that.
You also mentioned about new content entering your game. So does Wizards need to put a stop in place because you can't take the time to say no and control what enters your game?
I've definitely walked into a board game store and said "Wow..that game has 8 expansions? I think you have to REALLY like that game to get into it, otherwise you're looking at a large investment for nothing." I've definitely considered whether to buy a game or not based entirely on how many expansions were available.
So, yes, I believe it is incumbent on Wizards to make sure to release splat books at a slow enough pace that it doesn't overwhelm my game. "Controlling what enters my game" is a lot of work. It requires that I read through the book in its entirety in order to make sure there aren't any hidden synergies between abilities that will make one too powerful if I allow another one in. Reading a book generally takes me about 5-10 hours of dedicated reading time. Which is why I haven't even finished reading the Monster Manual or DMG yet. When my players start asking me if they can be stuff from the new book starting the week after the book comes out, I need to push aside things I'd rather do to make time to fully read and understand the book.
I haven't actually been doing that. I've been relying on WOTC to keep things balanced and just allowing everything. Unfortunately, that has come back to bite me in the past. Books coming out too quickly meant a lack of quality control on WOTC's end. I hope that books coming out less often will mean that'll be less of an issue.
However, most of the time the problem doesn't come from just one book. The problem comes from having too many options. Options interact with each other and the more interactions there are the more likely one combination of options will come out overpowered. Keeping the options low means lowering the chance this will happen. This is one of the major reasons I'm enjoying 5e right now. The lack of options means I'm not having rules arguments about weird interactions between stuff from 3 different books every session. Also, the low number of options is keeping my player's power gaming tendencies in check. People aren't constantly discussing their builds and the builds of the next 3 characters they plan on playing when this one dies.
Adding a lot of material will break that.
Loads of products don't keep people from playing the game. History really hasn't shown this nor does the present. I don't really see anyone complaining about coming into the Drizzt novels, or the Dresden Files. If you haven't read those books before then how would you know where to start? The Drizzt series has tons of books, but yet there are still new readers coming on board.
You haven't been the same places I've been then. Nearly every thread I've ever read about the FR has a bunch of people showing up saying they hate the Realms because they feel overwhelmed by the amount of information available for it and they feel the need to know it ALL but it would take years of reading to read through all of it so they avoid it.
There will always be people willing to put in that effort because they are really dedicated. But most won't.
The problem I see right now is Wizards failed to put in an appropriate introduction in the PHB. When you open Homeland, you will see a list of the next few books in the series in order to give you a starting point so you will know where where to go after you've finished that first book. Same goes in the other Drizzt series books. Wizards failed to do this with 5th edition. Page 1 should have been a list of the three books a group needs to purchase in order to begin their journey. That way, a parent coming in can flip open the book to page one and see that they would probably need to buy the DMG and MM on top of the PHB.
Wizards doesn't want to give people that impression, though. Most players get into the game by joining an established group. Which means they aren't DMing and don't need a MM or a DMG. They likely enjoyed the game enough that they want their own PHB to reference during the game. Then they "graduate" to DMing in a year or two after they start playing. Then they get the extra books. By that point, the players should be very familiar with what books they need.
That's how I started playing D&D back in the day. I joined a group of people I met on BBSes and a couple of weeks later, I bought a PHB. I didn't get any more books for a year or two. Then, I went a little crazy.
Still, WOTC wants there to be a path to start the game at around $50 without insinuating that you ever need to buy another book. They want the choice to be between that new video game for $60 dollars and being able to play D&D for $50.
Each book that is on the shelf is another reminder that you need to spend more and more money to "complete" your D&D game. Which is a barrier to entry for a lot of people.
Sure, the vast majority of the people here are mega nerds who are fairly obsessed with D&D and therefore the idea that someone would be turned off by more awesome books seems completely alien to us, but I remember when I first started being really paranoid about not being able to play the game "properly" if I didn't buy ALL the books.
