To be fair, you can obtain all Pokémon from Bulbasaur all the way up to Arceus without having to transfer from Gen III.
In fact, the only Pokémon you couldn't obtain between D/P/Pt were the Gen I/II/III starters and most legendary Pokémon.
Then HG/SS came and you could get the weather trio + Latias/Latios, while Platinum let you get Regirock/Regice/Registeel (Though it's true you needed an event-only Pokémon for that. At least you could trade the event Regigias on the GTS. :/)
Emm... You mean Rayquaza?
Maybe I didn't do that extensive research on that part, so my bad.
But I simply don't get the DS lite logic.
Why would I want to buy a much weaker system that plays DS games when I can play 99% of them just fine on a 3DS?
I just don't see why would people want to do that. It's a waste of money.
Maybe you can buy one for a very low price in the US or preowned/black Friday or whatever but some places in the world have to pay a lot more for a DS Lite, even if its preowned.
Kids... Would their parents allow them? Are you willing to take the risk if it were a preowned copy?
I wouldn't fork out extra cash for a weaker system (that my next gen set can mostly do) just for that one feature, unless I can get a DS Lite for less than the price of a regular game from where I'm living.
Plus, even if I were to complain how "awful" DS games play on the 3DS, I could have gotten a DSi XL or DSi (if you want portability) instead of a regular DS Lite. I am going to assume only a minority will go for the DS Lite route.