Not much because they choose old pokemon on many factors, and what past generation they are from is probably the last thing they consider if they even do it.
I mean, for example they would put a Wingull in a game because there is locations where wild Wingulls just make sense, not because they want more gen 3 pokemon.
Besides, each generation has a different approach and might deal differently with the inclusion of old pokes.
-So Johto for example was basically a second Kanto (regarding what common pokemon are in it), with some new hidden and rare pokemon you have to use new methods to find.
-Hoenn wanted to be a fresh start, but didn't dare to alienate the fans by not including iconic classics... and a specific example is that it had three of the trade evolving ones, since they didn't introduce more than one new. Of course they would use more gen 1 than gen 2 here, because gen 2 was extra pokemon, but they needed casual average pokemon like Magikarp, Zubat and Oddish. ...And then a Slugma because they actually had a fitting location for it this time. There is tons of reasons for certain pokemon to appear, and its not about gens.
-Sinnoh was very much like Johto, as in that it had only a handful of casual pokemon, so it resorted on the use of anything fitting from the past again to fill the gaps for most areaa in the game.
You say there is 34... but I count like less than 20, so I assume you were looking at Platinums upgraded version though. Of course they had to put in everything they evolved for that, which is the case for the initial DP dex too btw, with most of the new stages being for Johto-mons. So you could say it had a noteworthy amount of those because it had evolutions/babies of them.
Also, the percentage barely takes the lead of gen 1 with ~30%, of which there are more casual ones in.
-BW depends on whether you mean the enw Unovadex for BW2 or postgame stuff in BW, which was full of random and just filled with everything they could add in kind of thing.
And again, gen V is done. There is no way on earth going to be more DS games. Starting new graphics with something new is quite safe to assume.