It was never officially confirmed or said that they would remake the Generation III games, and it was never reall a trend to begin with.
Pokemon R/G/B (Generation I) and Pokemon G/S (Generation II) were remade because they were very old games on out-dated systems; they just wanted to bring it back up in higher quality and definition so people didn't have to play it on old systems (or deal with the tons of bugs in the games alone).
Although this entire answer is only an assumption, they may just remake the Generation III games when they get very old, and have an anniversary for that game or something.
(The Gameboy Advance systems were retired and stopped in production, so eventually, it will get to be as old as the old Gameboys were to us now; but there was nothing really wrong with Pokemon R/S/E, so they may not remake it and just leave it as classics.)
No one really knows what Nintendo is planning, for now at least.