They seem to have the idea that making alt characters should require just as much of an investment, or slightly less than your main character. You can speculate as to why they take this approach, whether it be something nefarious like trying to increase play time, or simply a philosophy they have in regards to every character requiring the same amount of work.
Most people I know don't "mind" systems like reputation, expansive quest lines, or other mechanics such as essences being in the game. What they don't like is having to do them multiple times, even if some of the requirements are eased quite a bit. My group of people simply want an easy way to pick up alternate characters, with essentially the only real barrier to entry being that gear is the only thing holding them back. Aside from reaching certain pillars in the game for the first time (PvP, raiding, M+, etc), most people I think would agree that one of the more enjoyable aspects of your main and alt characters, is the actual growth through gear.
If we assume that Blizzard has nefarious logic in regards to their view of alts, it's a pretty retarded stance for them to have. If they honestly think that playtime will increase because alternate characters have to do some portion of essence, AP and questlines (or all) as your main, they're sorely mistaken. What ends up happening is people don't even bother making an alt because there's a monumental task set forth for them to get that character up to snuff, and it's always something lingering on your mind. The gear grind is fun, but knowing that you're weeks behind on potential reputation grinds, AP and the newly introduced essence system makes the experience of playing an alternate character fairly sour.
This isn't actually a new phenomenon. There were periods in the game where pretty much nothing mattered but your gear, and during those eras it was fairly difficult to gear your character, but most people were okay with that. WoD/MoP started introducing aspects that made you feel behind (the legendary items), and Legion/BFA really accelerated this to levels far beyond both WoD/MoP with both legendary items, AP, and other systems that make catching up an alternate character a pain in the ass.
In my opinion playtime can only be increased by making alternate characters very easy to transition too. Reputation, essences and even AP could be account wide. All of those systems are far more bearable (even AP/Rep) if you only have to do them once, and I don't think anybody would argue that most of the essences are okay to great to get the first time. If they made that compromise they could even reign back the shower of loot that we've been getting slowly over the past several expansions, and I doubt anybody would care.
TLDR; Make most systems account wide, or have massive catch up mechanics in place for alternate characters. The only difference between you main/alt should be the time investment you put into them in regards to getting gear.