They're already pushing it enough as it is. The N3DS won't let you do a system transfer to an Old.
And the Old XL is not that old. There are lots of users who just bought one, and wouldn't be thrilled if Nintendo neglected the Old platform. Also, the Amiibo reader for the Old is scheduled for release in a few months.
My guess is, at first, barring a couple of truly exclusive titles, newer games will be playable on the Old but might have extra features only available on the New. From then on, we will see either more N3DS exclusives or more Old-compatible games, depending on sales.
It will be just like other portable revisions like the Game Boy Color and the DSI. The GBC did well, so lots of GBC-only games came out. The DSI didn't, so publishers had to keep DS compatibility to make a profit.