QR has recently bought 1723, 1724, 1725, 1732, 1738, 1744, 1745, 1746, 1754 and 1765 from Aurizon. AIUI one more is going to get a Kuranda livery and the others will mainly operate infrastructure trains plus a bit of shunting at Mayne. There are GM diesels 20 years older than the last 1720s still in service south of the border so they are going to be around for a while yet.
I'm told there is also an agreement that 1770 will be sold/transferred from Auziron to QR Heritage when no longer required.
As to a possible replacement, something like the new 1600hp gensets bought by Pacific National for Bluescope Steel workings could be a possibility. Especially where they might spend a lot of time waiting around doing nothing and/or moving at low speed.
Cheers
David