We can, but the more general question is whether we should keep any compatibility/legacy code in major releases, or always force users to upgrade the code in breaking places (documenting upgrade thoroughly) and keep the codebase clean. It's hard to compromise between these options, as it's a half-measure. Full measure on the compatibility side would lead to OpenLayers-like API hell, and full measure on the other side is pretty acceptable for Leaflet in my opinion. That's one of the reasons why I didn't release 1.0 yet BTW — I want the API to mature first.