They didn't want to make new designs. That's it.
While its maybe easy to make a nicer sprite or pose, or maybe redraw them, aging them means changing their design. (Also all of the new sprites have pretty lazy poses.) Because if they're going to age them they really shouldn't be wearing the same clothes especially when teenagers/children are becoming older. They just didn't want to do it to all those old characters.
So I really wouldn't take it into any account when figuring out timelines.
Although it would've been cool to have seen all the old gym leaders have new designs and poses, they just thought it was too much work and left them all the same from HG/SS and D/P/PT except the R/S/E ones (cuz they were kinda outdated by all the others)