The main obstacles to repairability in such devices are intentional: part serialization, lack of documentation, and so on. Those don't help making the device any more compact or easier to manufacture, it's pure greed.
Address those problems and you can happily have your ultra-slim, tightly integrated laptop. It may be slightly less repairable, but as long as repair isn't intentionally being prevented, life will find a way.
Whether repair of such devices is economically viable is one thing and that's up to the market to decide, but making repair intentionally harder is a choice of the manufacturer and has nothing to do with how slim the laptop is.
You can have a somewhat repairable laptop even if it's slim and tightly integrated, and you can also have a completely unrepairable one even if all components are modular and accessible but then use strong cryptography to authenticate to each other.
Form factor is not the primary reason current tech is hard/impossible to repair, though the industry loves that people believe so, since it diverts attention from their intentional efforts to hinder repair.
Nowadays? A techpriest that can take apart Apple's iPhone stacked PCB assemblies, replace large BGA components in there, and then put them back together and have it work is a rare specimen. And "rare" means "expensive".
A hour of labor of someone who does neurosurgery on electronics isn't going to be cheap.
Not that Apple has any good reasons to make it even harder on the madmen who attempt and learn such repairs.
Things like PCB manufacturing? Putting those BGA chips where they go? Done entirely by machines.
Now, a notable exception to this rule is the "rework" or "remanufacturing" lines - where actual human specialists take devices that failed QC, or used devices, diagnose them, and bring them up to standard.
Those can be very involved. But official manufacturing still has strict limits on how far are they willing to go - and unofficial refurbishment lines have them beat on repair complexity.