Nothing (other than cost) stopping individual sprockets being on an alu carrier to spread load and still be replaceable. The higher end Campag cassettes have carriers with pairs of sprockets on, which is almost close enough, but then they also have deeper splines on their freehub/sprocket interface anyway…
About half the sprockets on the XT 780 11-36 were actually replaceable (and at a fairly reasonable price)
If you then used an expander then that was also replaceable.
But its down to cost…. the bigger sprockets are heavy in steel and fragile in alloy and presumably horrendously expensive in titanium so do you want a cassette that’s all replaceable but costs £200 or semi replaceable and costs £60 ??
Equally the Novatec freewheel replacement including bearings is £35-£40 so cheaper than a cassette…
(Prices for the D772 on my kids bike)
In many ways its like BB’s … the Deore External is £15 ish (less than new bearings for a Hope one) so its a matter of light/strong/cost again.
As Nober say’s we all moan about forced new standards … so if SRAM/Shimano are essentially defacto standards then I’ve no problem with other companies making non-standards… (for debatable value/benefit)