Friends of mine, new to the game, becoming discouraged by STORY QUESTS

[Niwashi]

I'm positive they will still require you to have HW on your account as Heavensward is required to reach level 60 and 4.0 is likely to increase the level cap from 60 onward and be designed with that increase in mind.

You contradicted yourself there. If (as I agree is most likely) the next expansion raises the level cap again and goes from 60 onward, then it too would give you a level cap raise, and HW will no longer be the only way to get one. ARR goes from 1-50, HW goes from 50-60, lets assume 4.0 goes from 60-70. That would mean someone with just ARR and 4.0 (but not HW) would have a level cap of 70 (which 4.0 gave them), but their content would go from 1-50, then have a gap before it picks up again from 60-70.

That wouldn't be the ideal way to play. Generally, they'd ask their friends "how do I get through this long empty patch from 50-60?" and be told to get HW so they can have a full expansion worth of fresh new content while they do it. If they take that advice, then nice for SE, they get another $10 or $20 bucks out of it that way. But if the player doesn't take the advice and keeps just ARR and 4.0, then they have to level from 50-60 on just the level 1-50 content. Quite a slog, but there is a lot of level 50 content, so it's certainly doable.