Not sure I agree with this.They've done these kinds of things in the past. The main set of nine modules for 4E all had a throughline and connective tissue, although no one would claim them to be one long storyline. They were nine separate adventures, but all had bits and bobs that premised the idea of an Orcus big bad, leading to eventually taking him on.
And I want to say Paizo did things similarly for 3E back when they were publishing Dragon Magazine and came up with their original ideas for "adventure paths". Things like the town of Cauldron and the modules that all centered around that... they were all individual stories but did "connect" together in minor ways.
I mean heck... you could also say the original Slave Lords and Giants series' of adventures for AD&D were the same kind of thing-- separate adventures that were meant to link into a longer campaign.
If you can accomplish a linking storyline between disparate adventures to give an idea of campaign flow, I think many DMs appreciate it. If it is there to use if you want but can easily ignored if you don't... it just makes a product (or series of products) have more tools in the box to be used.
There was nothing tying the Slave Lords to the Giants series. They're in completely different locations for one - as in REALLY far apart. Deal with completely different issues and about the only reason that people do link them is because of the levels - A4 ends at about the right character level to start G1. But, as written, these are completely separate adventures with nothing tying them together.
Cauldron is a straight up AP. Every adventure leads you directly to the next one. You generally were never expected to mix the AP at all - Shackled City and Age of Worms don't really connect at all and there certainly isn't any idea that you are supposed to mix and match them. They had the odd side trek adventure for the AP's, but, the adventures in the AP were very much directly linked and not linked to anything outside the AP.