I disagree. The complaints about the lack of Primaris customization is legendary at this point. Primaris do carry a philosophy that differs from classic marines in that they limit wargear flexibility.
There aren't Primaris Gravastators or whatever other weapons you referred to - just Hellblasters for heavy infantry. There aren't three different generalist units that can fit in anti-horde elements - for Primaris, it's all a specialist unit's job, and they don't do any other job. An Intercessor squad doesn't have access to anti-armour elements - it's just bolter marines. A Hellblaster squad doesn't get access to heavy bolters for anti-horde - it's all just plasma. Reivers can't supplement their anti-light infantry role with a power fist or meltagun for flexibility against bigger targets. So on and so forth.
Ishagu makes sense here. Primaris provide a helpful new design space built on the 30k legion concept. Focused purpose at the cost of flexibility. That is a much, much easier environment to balance with points.
You've missed my point: the roles of the units aren't different. The lack of customisation is pretty irrelevant.
Devastators are heavy weapon units with, predominantly, anti-armour weapons.
Assaults are light infantry killers.
Tacticals are a flexible line-holder.
Primaris follow ostensibly the same pattern:
Hellblasters are anti-armour.
Inceptors are, predominantly, anti-horde with an option for anti-armour.
Aggressors are anti-horde.
Intercessors are a line holder with an anti-horde bent.
I would like to point out that I'm not advocating for Primaris to get endless customisation: I am saying that the roles that their units have been designed for is not actually that different to how regular Marines do it. The difference is that Primaris don't have the option for switch, for example, a Hellblaster squad into a volume-of-fire anti-horde unit like Devastator can.
Functionally, regular Marine units have roles same as Primaris. The key is that the rules don't actually do much to support Marines either way.
My main response is that Ishagu's assertion that Primaris are the only way to make Marines better going forward is wrong; because the reason Primaris are even inserted is because GW hasn't made regular Marines actually good enough, which has nothing to do with the roles that they play, simply that the model/stat/weapon/points balance is off.
It may be the case that it's easier to balance a unit with no customisation options: I would counter that with the example of Reivers and Intercessors. Both units have the same weapon, functionally (24" A2 4/0/1) but Intercessors cost more - why? Isn't that exactly the kind of role overlap that is the issue with regular Marines? Why ever take Auto Bolt Rifle Intercessors, considering that they fulfill the exact same role as Reivers but cost more? They're a functional overlap which is identical to the issue with Assault Marines/Vanguard Veterans.
That's the crux. Primaris aren't, functionally, any different from regular Marines, they simply have less options currently - which begs the question of why bother at all, why not just make regular Marines actually work properly instead of trying to replace them with 'totally different' units.