Grumpy old men
They still sell 2x4 bricks and boxes of basic bricks. Expensive? They last forever at least.
I suppose Lego could have stuck to just that, but they'd have 200 employees and one factory.
We quite enjoy the tie-in sets, the TV series and the computer games (and the theme parks and even the hotel). What we have now is a lot of choice. Too diverse a product range might be a problem for Lego but it's hard to see how it's a problem for its customers.
"Too many custom parts" isn't quite true either. Having bought sets from many themes there are hardly any brick-types that aren't seen used in different ways across multiple sets. There are a lot of different pieces, though, and this does make it harder to build creatively with bits from disassembled themed sets. The solution is to buy boxes of basic bricks.