Killzone was always going to be the standout launch title out of any of the next-gen systems this fall.
Losing DriveClub definitely sucks, but it wasn't what I was really looking forward to at launch.....nor was it really something that was going to sell much anyways. The genre is basically dead at this point and casual audiences really don't care about racing titles outside of Gran Turismo and Mario Kart.
If it makes it a better game, it's for the better. There's also bound to be a PS4 exclusive drought after InFamous: Second Son.
An April -- May launch may do wonders for the game's quality.
Launch lineups don't need massive amounts of AAA titles...the best launches have generally only had one must have game -- think Mario 64, Halo, Soul Calibur, and so forth.
So, if Killzone delivers, that's all Sony needs and all I was really expecting for a solid launch in terms of exclusives. I'd take one Killzone over all of the Xbox One's launch lineup which does nothing for me.
There's still tons of third party content and indies for this year, as well.
I doubt Sony will give away Knack to PS+ members, although it would certainly do a lot of goodwill to fill the DriveClub void at launch. They're already giving the title away in Japan, it probably wasn't a title with a massive budget anyways, and giving it away for PS+ members in all regions could certainly boost PS+ memberships (perhaps more than DriveClub).
At a bare minimum, a discount on Knack for PS+ members would probably get me to pick the game up since I'm on the fence about it now....A PS+ price of $29.99 would be pretty good.