Sollozzo recognizes that drugs are the way of the future and that Vito "pensa all'antiqua." It is logical that, with Vito out of the way, the rest of the Family would come along.

In fact, Sollozzo himself indicates that Tom, not Sonny, was really the key. He realizes that Tom knows it was "the right thing to do" and expects Tom to convince Sonny to go along.

Plus, Sollozzo couldn't be expected to just give up once he got Vito's refusal to help. Obviously, he would have done something.

So I think the hit happens either way.

BTW, I also agree with mustachepete that Caan kind of blew the delivery of the line. He should have sounded more interested than belligerent.