I think we're all not thinking about this in the correct way, so here is the way I see it. I don't think a content-lock would have been put in place without some serious thought put behind it, so let me explain.
Everything we've done in this game has always been finely tuned, refined & catered toward players of a specific ilvl range. Everyone seems to forget that ilvl requirements have been a thing in this game for a really long time, and no one seemed to care about that.
We've also had 5 major content updates, all raising the bar at the same level cap of 50. Obviously there is going to be a HUGE gap in someone who is i130 versus someone who is a fresh 50 in i50-60 gear. So lets take this i50-60 geared individual, lets throw them in a 2.4/2.5 dungeon or what ever and now lets see how they deal with content not fully understanding their skill-set, rotations, or even simple mechanics. They can't even do Titan HM in that gear let alone stay alive due to lack of experience with said mechanics.
Now think of HW in the same light, do you really think a fresh 50 stepping foot into Ishgard / exploring new HW content with this skill-set & gear is a good idea? Most of them would be miserable if anything. I would presume HW has been catered & balanced around Steps of Faith ilvl requirements (which is around i90).
So how did SE decide to get around this? Sure content-gating is a thing, but for good reason. It has been said numerous times how SE plans to get around this. For one, main-scenario quest will yield a much higher exp gain than ever before. This means newer players will be stepping into HW content above the current level cap of 50. All the main-scenario quest are now also going to yield meaningful gear rewards as well, so when those new players do finish 2.55 main-scenario quest they will be more than ready & prepared to meet the challenges of HW.
This route also prolongs the lifespan of the current content that will now be obsolete to most of us by the time HW launches. I've had a hell of an experience on this game when 2.0 relaunched, I would hope that others get to have those experiences as well. A new 50 joining the game as it is now has an IMMENSE amount of content available to them as it is now. You should value the experience of newer players, they've not seen or played through the things we have. If we allow them to bypass all of that would you really want those players running the same dungeons & potentially raiding with you?
So what about the current level 50s that are returning after having quit? Honestly I don't think it will take long for them to power through main-scenario quests. Sure some may not make it by HW launch, but I also don't understand what the rush is. No real meaningful content will be available to us until 2 weeks / 4 weeks post HW launch. Everyone is going to have plenty of time to prepare and get ready for it. If they/anyone are so concerned about being that first level 60 on a new class, guess what, this wouldn't be a problem for them then would it?
Now what about the new jobs also being locked? Well for one all the new classes start at level 30 but require access to Ishgard to unlock them, meaning a player will have to be level 50 at the minimum to even access them, that being said do you expect someone completely unfamiliar to the game to be able to pick up and play a level 30 class? I wouldn't think so, and neither should any of you.
Anyway thats my two cents.