They probably won't have any use for the GBA games once HG/SS is released. I think that if they make a new console though, and a 5th Generation, then Nintendo may re-use/copy the GBA slot + DS function in a new way. Like if you have a DPP or HG/SS game in the DS slot on
(insert new game system/handheld here), then you can migrate DPP/HGSS Pokemon over to the 5th Generation game. That may be it for the GBA games though. The GBA games were only used for migrating in DPP, so it's very likely that they won't be used for anything else.
I totally agree, The pal-park is still in HG and SS though. That will most likely to be for the Hoenn pokemon.
The thing is aswell, when the 5th Gen comes out (There will be one, the pokemon games are still going strong) i think it will be shocking to seethe dofference from the original RBY games to the 5th gen