I suspect Dragonlance provided a much bigger push in the AP direction than did anything else in the early days.
Slavers - GDQ predates DL.. not by a lot... but both are progress in that direction. From different sources. But, same time frame, there were others. DL1 is 1984. Q1 is 1980. Different points in the timeline.
Note that the Slavers and Giants portions run just fine separate from each other; D1-3 work fine without the Slavers or GIants portion... but QotDP is a direct follow-on to D3. It's not the same as a more modern adventure path...
And DL? Sure, it's one big campaign adventure... railroaded all to hell... but fun for some.
Other companies were starting to experiment with similarly long-form adventure content...
GDW released a campaign length work, entitled,
The Traveller Adventure... in 1983. It's been retitled in reprint to
Aramis: The Traveller Adventure - a serious misnomer since it's not really focused upon the Aramis Trace, let alone the subsector capital Aramis. (And, despite my being a GDW fanboy and admin at COTI, Aramis as a handle is a reference to the Musketeer, not the world, trace, nor subsector in the Marches.)
1985, we get the serialization of the DGP's "Grand Tour" adventure in
Traveller's Digest.
1986, GW starts serializing The Enemy Within.