I still feel like Miste has a point.
Let me explain you why, let's say that we take onto your gearing structure and add more middle man into the process.
Which is going to look something like this :
* Entry (Expert/Tomestone 315)
* New middle man (335)
* EX Primals (With added loots to cover each slots - 355)
* Savage/Ultimate (375)
Now we've solved the problem that you've got with progression, since you will need to grind onto each tier before moving onto the next (For most of the player base that is) but you're not addressing your other concern, which is gear revelance and I'll try to adress why (And even come up with possible solutions !), since you want an never ending incentive to actually gear up, let's say that the new shiny patch is out, let's call it 4.2 for now.
Here are what the rewards will look like (Arbitrary still) :
* Entry (Expert/Tomestone 375)
* New middle man (395)
* EX Primals (415)
* Savage/Ultimate (435)
Which would end up with the same problem that we have now.
Alternatively, I know you could choose to lower the tiers between each (+20 instead of "starting from scratch") but, if you're only going to tackle one of those difficulties in mind, it would also lower the gap of what you're winning between each tier, which would also lower the incentive to tackle upon said difficulty (unless you want to grind all those stairs), someone (Entry runner) that was going to be trying to do the "New middle man" from my former example could actually ask "Why ?" since he already know that it'll be outdated in 3/6 months time.
There's an interesting argument to be made there by saying that it would encourage "improvement" by itself by putting artificial walls (a.k.a ilevel) in front of it.
By the end of an expansion, your entry ilevel could be at the level of your former savage ilevel which would get reset as soon as you put a foot into the new expansion, getting me to my conclusion (and personal opinion) that fun should be put first and foremost, my entire post isn't going to address everything possible outcomes that would come with such a change (such as "Why should I have these tomestones now?" or "Where does the crafting gear fits in there ?") and I kind of admit that I kinda "hi-jacked" my personal purpose upon writing it (Bad, I know, I know...).
P.S: I actually like the current system more since it doesn't deny me from trying any of the content without grinding all of those artificial stairs, so to speak.
Tl;dr If you actually don't like the content that you're currently doing for what it is there's always going to be a moment where your gear get outdated, it's just a matter of time. (Even in a vertical (or horizontal ?) progression game ? I think that's how you guys calls it.)