Dawn's departure must have derailed the plans around the EV3 album somewhat, but looking at the release dates, there's only 5 months between the release of 'Don't Let Go' and 'Whatever' - a bit longer than usual perhaps, but not enough in itself to kill off the momentum, particularly as 'Don't Let Go' was from a film rather than marketed as the first single from the forthcoming album.
The bigger problem I suspect was that 'Whatever' wasn't strong enough to be a new 'lead' single for the album (even if some of the remixes were pretty good). They'd generally only managed one smash per album ('Hold On' and 'My Lovin') up to then anyway, so maybe they'd used that up already on 'Don't Let Go'.